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Stockholders' Equity - Summary of Activity for Outstanding Three-year PSU Awards (Detail) - PSU Awards
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]  
Outstanding at beginning of period 827,712
Granted 300,002
Converted (325,178)
Forfeited or expired (139,605)
Outstanding at end of period 662,931
Aggregate Intrinsic Value, converted | $ $ 30.0
Fully vested at end of period 543,975
Aggregate Intrinsic Value, Fully vested at end of period | $ $ 26.8
Maximum  
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]  
Performance units granted percentage 200.00%
Minimum  
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]  
Performance units granted percentage 0.00%