| Stock-Based Compensation Plans - Components of Stock-Based Compensation Included in Consolidated Statements of Net and Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | |
|---|---|---|
| Mar. 31, 2014 | Mar. 31, 2013 | |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Share-based compensation expense - Independent contractors | $ 487 | |
| Allocated share-based compensation expense | 717 | 436 | 
| Restricted stock and SARs (prior to IPO) [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Allocated share-based compensation expense | 436 | |
| Restricted Stock [Member] | Non-employee directors [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Allocated share-based compensation expense | 30 | |
| Restricted Stock Units (RSUs) [Member] | Employees [Member] | ||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Allocated share-based compensation expense | $ 200 | |