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Equity-Based Compensation Plan - Summary of Performance Unit Award Activity (Detail) - Performance Shares [Member]
3 Months Ended
Mar. 31, 2018
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, 2017 2,536,594
Granted 0
Settled 538,594
Forfeited (50,000)
Outstanding at March 31, 2018 1,948,000