Equity - Treasury Stock (Detail) - USD ($) shares in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Equity, Class of Treasury Stock [Line Items]
Repurchase of common stock	$ 15,993,000	$ 0	$ 189,906,000	$ 0
2017 Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired (in shares)	0.6		5.6		1.4
Treasury stock acquired, value	$ 20,000,000		$ 193,900,000		$ 50,000,000
Repurchase of common stock	16,000,000		189,900,000
2017 Repurchase Program | Maximum
Equity, Class of Treasury Stock [Line Items]
Aggregate authorized amount of common stock to be repurchased		300,000,000		300,000,000
Treasury Stock Associated with Deferred Compensation Plans
Equity, Class of Treasury Stock [Line Items]
Treasury stock acquired, value	$ 0	$ 200,000	$ 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, value			$ 14,200,000	$ 17,800,000