Share Repurchase Programs (detail) - December 15, 2015 [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Schedule Of Share Repurchase Programs [Line Items]
Share Repurchase Authorization Date	Dec. 15, 2015
Amount authorized to be repurchased	$ 1,000.0
Remaining authorized repurchase amount	$ 527.0