Capital Stock - Share Repurchase Programs (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Schedule Of Share Repurchase Programs [Line Items]
Remaining authorized repurchase amount	$ 1,324.3
December 15, 2015 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Amount authorized to be repurchased	1,000.0
Remaining authorized repurchase amount	$ 324.3
Share Repurchase Authorization Date	Dec. 15, 2015
October 22, 2018 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Amount authorized to be repurchased	$ 1,000.0
Remaining authorized repurchase amount	$ 1,000.0
Share Repurchase Authorization Date	Oct. 22, 2018