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Stockholders' Equity (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2020
Sep. 30, 2019
Feb. 07, 2018
Dec. 31, 2017
Dec. 31, 2019
Feb. 13, 2017
Dividends, Common Stock [Abstract]                  
Dividends Payable, Date Declared Aug. 05, 2020 May 13, 2020 Feb. 27, 2020            
Dividends Payable, Amount Per Share       $ 0.075          
Dividends Payable, Date of Record Aug. 20, 2020 May 28, 2020 Mar. 13, 2020            
Dividends Payable, Date to be Paid Sep. 04, 2020 Jun. 12, 2020 Mar. 30, 2020            
Stock Repurchase Program, Authorized Amount                 $ 300.0
Stock Repurchase Program, Period in Force           3 years      
Treasury Stock, Shares, Acquired             133.5    
Stock Repurchase Program, Remaining Authorized Repurchase Amount $ 166.5     $ 166.5          
Payment, Tax Withholding, Share-based Payment Arrangement     $ 32.1 $ 35.5 $ 37.7        
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]                  
Preferred Stock, Shares Authorized 100.0     100.0       100.0  
Preferred Stock, Shares Issued 0.0     0.0       0.0  
Preferred Stock, Shares Outstanding 0.0     0.0       0.0  
Dividend Paid [Member]                  
Dividends, Common Stock [Abstract]                  
Dividends, Common Stock, Cash $ 14.3 $ 14.3 $ 14.2            
Restricted Stock Units (RSUs) [Member]                  
Dividends, Common Stock [Abstract]                  
Vesting of restricted stock units     0.9            
Share-based Payment Arrangement, Shares Withheld for Tax Withholding Obligation     0.3            
Stock, Shares Issued Net of Shares for Tax Withholdings     0.6