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Stock-based Compensation Plans - Additional Information 2 (Detail) - USD ($)
$ / shares in Units, $ in Millions
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Restricted Stock Equity Awards    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Granted 1,785,446 875,000
Granted, weighted average grant date fair value $ 17.00 $ 32.93
Vesting period 3 years  
Stock-based compensation expense $ 12.4 $ 12.5
Restricted Stock Liability Awards    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Granted 806,129  
Granted, weighted average grant date fair value $ 15.25  
Stock-based compensation expense $ 10.5 $ 9.0
Restricted Stock Liability Awards | Employees    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Granted 675,000 449,000
Granted, weighted average grant date fair value $ 15.22 $ 26.18
Vesting period 3 years 3 years
Restricted Stock Liability Awards | Non-Employee Directors    
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Granted 131,000 90,000
Granted, weighted average grant date fair value $ 15.46 $ 25.01