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Stockholders' Equity - Additional Information (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
6 Months Ended 125 Months Ended 131 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]      
Treasury stock, total repurchase during period (in shares)   11,142 12,513
Treasury stock, average price of share repurchased (in dollars per share)   $ 27.57 $ 26.94
Treasury stock, total cost of repurchased stock   $ 307.2 $ 337.1
Stock repurchase program, remaining amount available for repurchase $ 112.9   $ 112.9
Performance Shares | Minimum      
Debt and Equity Securities, FV-NI [Line Items]      
Performance awards, percentage of grant available to vest 0.00%    
Performance Shares | Maximum      
Debt and Equity Securities, FV-NI [Line Items]      
Performance awards, percentage of grant available to vest 100.00%    
Employee stock purchase plan      
Debt and Equity Securities, FV-NI [Line Items]      
Shares reserved for grant (in shares) 1,265   1,265
2003 Plan | Employee stock options      
Debt and Equity Securities, FV-NI [Line Items]      
Shares reserved for grant (in shares) 3,600   3,600