Quarterly report pursuant to Section 13 or 15(d)

Equity - Treasury Stock (Detail)

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Equity - Treasury Stock (Detail) - USD ($)
shares in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Jun. 30, 2017
2018 Repurchase Program            
Equity, Class of Treasury Stock [Line Items]            
Aggregate authorized amount of common stock to be repurchased $ 500,000,000.0   $ 500,000,000.0      
2017 Repurchase Program            
Equity, Class of Treasury Stock [Line Items]            
Treasury stock acquired (in shares) 0.7   6.3   1.4  
Treasury stock acquired $ 23,800,000   $ 217,700,000   $ 50,000,000.0  
2017 Repurchase Program | Maximum            
Equity, Class of Treasury Stock [Line Items]            
Aggregate authorized amount of common stock to be repurchased           $ 300,000,000.0
Treasury Stock Associated with Deferred Compensation Plans            
Equity, Class of Treasury Stock [Line Items]            
Treasury stock acquired $ 0 $ 0 $ 3,300,000 $ 3,400,000    
Common Stock Withheld for Settlement of Employee Tax Liabilities            
Equity, Class of Treasury Stock [Line Items]            
Treasury stock acquired (in shares)     0.4 0.5    
Treasury stock acquired     $ 14,700,000 $ 18,200,000