Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member] |
9 Months Ended |
|---|---|
|
Sep. 30, 2015
shares
| |
| Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] | |
| Outstanding at December 31, 2014 | 924,143 |
| Granted | 597,249 |
| Outstanding at September 30, 2015 | 1,521,392 |