Shareholder's Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Dec. 06, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 19, 2017	Dec. 31, 2014
Shareholders Equity
Common stock, shares authorized		426,000,000	426,000,000
Common stock, par value (per share)		$ 0.01	$ 0.01
Common stock shares repurchased, shares		3,000,000		3,100,000
Stock Repurchased During Period, Value		$ 200.0		$ 200.0
Dividends payable amount per share					$ 0.350
ApprovedNotDeclaredlDividends	$ 1.40
Dividends payable		$ 63.1	$ 61.8
Dividends paid per common share		$ 1.380	$ 1.34	$ 1.28
December 2014 Share Repurchase Program [Member] [Domain]
Shareholders Equity
Authorized amount to repurchase shares of common stock						$ 1,000.0
Treasury Stock [Member]
Shareholders Equity
Stock Repurchased During Period, Value		$ 0.0
Treasury Stock, Shares, Retired				(19,100,000)
Treasury Stock, Retired, Cost Method, Amount				$ (1,210.9)