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Stock-Based Compensation - Summary of Employee and Director Restricted Stock and Restricted Stock Highlights (Detail) - Employee Restricted Stock Units and Director Restricted Stock Units - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Fair value of vested restricted stock units $ 3.5 $ 5.6 $ 18.7 $ 9.9
Tax benefit recognized from vested restricted stock units $ 0.8 $ 1.1 $ 3.4 $ 2.1