Quarterly report pursuant to Section 13 or 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 9 Months Ended
Aug. 31, 2018
May 31, 2018
Mar. 31, 2018
Feb. 28, 2018
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested shares at December 31, 2017         981,855
Granted 74,058 31,986 15,000 126,712 247,756
Vested         (322,463)
Transferred         0
Forfeited/canceled         (7,704)
Nonvested shares at September 30, 2018         899,444
Unrecognized stock-based compensation expense as of September 30, 2018         $ 22,031
Weighted average remaining vesting period (years) as of September 30, 2018         3 years 2 months 19 days
Nonvested weighted average grant date fair value per share, beginning balance         $ 23.90
Weighted average grant date fair value per share, Granted $ 0 $ 0 $ 0 $ 0 34.92
Weighted average grant date fair value, Vested         22.06
Weighted average grant date fair value, Transferred         26.52
Weighted average grant date fair value, Forfeited/canceled         28.76
Nonvested weighted average grant date fair value per share, ending balance         $ 27.56
Restricted Stock Awards [Member] | Non-Employee Directors [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested shares at December 31, 2017         30,732
Granted 0 12,852 0 0 12,852
Vested         (16,488)
Transferred         0
Forfeited/canceled         0
Nonvested shares at September 30, 2018         27,096
Unrecognized stock-based compensation expense as of September 30, 2018         $ 526
Weighted average remaining vesting period (years) as of September 30, 2018         11 months 12 days
Restricted Stock Units [Member] | Employees [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested shares at December 31, 2017         500,859
Granted 10,407 4,854 15,000 106,419 136,680
Vested         (142,433)
Transferred         (7,356)
Forfeited/canceled         (1,960)
Nonvested shares at September 30, 2018         485,790
Unrecognized stock-based compensation expense as of September 30, 2018         $ 10,884
Weighted average remaining vesting period (years) as of September 30, 2018         3 years 3 months 7 days
Restricted Stock Units [Member] | Independent Contractors [member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested shares at December 31, 2017         450,264
Granted 63,651 14,280 0 20,293 98,224
Vested         (163,542)
Transferred         7,356
Forfeited/canceled         (5,744)
Nonvested shares at September 30, 2018         386,558
Unrecognized stock-based compensation expense as of September 30, 2018         $ 10,621
Weighted average remaining vesting period (years) as of September 30, 2018         3 years 3 months 7 days