Shareholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions		3 Months Ended	12 Months Ended
	Dec. 05, 2017	Mar. 31, 2018	Dec. 31, 2014	Feb. 27, 2018	Dec. 31, 2017
Stockholders Equity Note Disclosure [Line Items]
Dividends Payable, Amount Per Share	$ 0.35			$ 0.35
Share repurchase (in shares)		2.2
Stock Repurchased During Period, Value		$ 150,000,000
ApprovedNotDeclaredlDividends	$ 1.40
Dividends payable		62,400,000			$ 63,100,000
December 2014 Share Repurchase Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Repurchase of shares of our common stock up to a maximum aggregate value			$ 1,000,000,000.0
Common stock authorized for repurchase, expiration date			Dec. 31, 2019
Share repurchase program remaining available amount		$ 450,000,172