Annual report pursuant to Section 13 and 15(d)

Stock-Based Compensation Plans - Outstanding Awards Under 2013 Omnibus Equity Incentive Plan (Detail)

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Stock-Based Compensation Plans - Outstanding Awards Under 2013 Omnibus Equity Incentive Plan (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Nov. 30, 2018
Aug. 31, 2018
May 31, 2018
Mar. 31, 2018
Feb. 28, 2018
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Nonvested shares at December 31, 2016           981,855 1,050,430
Granted 10,322 74,058 31,986 15,000 126,712 258,078 264,550
Vested           (333,724) (295,597)
Transferred           0 0
Forfeited/canceled           (14,634) (37,528)
Nonvested shares at December 31, 2017           891,575 981,855
Nonvested weighted average grant date fair value per share, beginning balance           $ 23.90 $ 22.38
Unrecognized stock-based compensation expense as of December 31, 2018           $ 18,926  
Weighted average grant date fair value per share, Granted           $ 34.94 26.86
Weighted average remaining vesting period (years) as of December 31, 2018           3 years 2 months 23 days  
Weighted average grant date fair value, Vested           $ 22.31 21.21
Weighted average grant date fair value, Transferred           30.69 25.70
Weighted average grant date fair value, Forfeited/canceled           30.17 23.30
Nonvested weighted average grant date fair value per share, ending balance           $ 27.59 $ 23.90
Restricted Stock Awards [Member] | Non-Employee Directors [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Nonvested shares at December 31, 2016           30,732 29,112
Granted 0 0 12,852 0 0 12,852 17,538
Vested           (16,488) (15,918)
Transferred           0 0
Forfeited/canceled           0 0
Nonvested shares at December 31, 2017           27,096 30,732
Unrecognized stock-based compensation expense as of December 31, 2018           $ 352  
Weighted average remaining vesting period (years) as of December 31, 2018           9 months 14 days  
Restricted Stock Units [Member] | Employees [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Nonvested shares at December 31, 2016           500,859 566,480
Granted 6,080 10,407 4,854 15,000 106,419 142,760 164,586
Vested           (146,122) (140,480)
Transferred           (23,755) (58,503)
Forfeited/canceled           (1,960) (31,224)
Nonvested shares at December 31, 2017           471,782 500,859
Unrecognized stock-based compensation expense as of December 31, 2018           $ 9,542  
Weighted average remaining vesting period (years) as of December 31, 2018           3 years 2 months 4 days  
Restricted Stock Units [Member] | Independent Contractors [Member]              
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]              
Nonvested shares at December 31, 2016           450,264 454,838
Granted 4,242 63,651 14,280 0 20,293 102,466 82,426
Vested           (171,114) (139,199)
Transferred           23,755 58,503
Forfeited/canceled           (12,674) (6,304)
Nonvested shares at December 31, 2017           392,697 450,264
Unrecognized stock-based compensation expense as of December 31, 2018           $ 9,032  
Weighted average remaining vesting period (years) as of December 31, 2018           3 years 4 months 20 days