Common Stock Repurchase Program (Detail) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended		3 Months Ended	6 Months Ended	9 Months Ended
	Oct. 31, 2018 $ / shares shares	Jul. 31, 2018 USD ($) shares	May 31, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) item shares	Sep. 30, 2018 USD ($) item $ / shares shares	Sep. 30, 2017 USD ($)
Accelerated Share Repurchases [Line Items]
Total repurchases					$ 750.0
Cash paid for repurchase of common stock					750.0	$ 750.0
Authorized share repurchases					$ 500.0
Accelerated Share Repurchase Agreement (ASR) [Member]
Accelerated Share Repurchases [Line Items]
Number of ASR agreements | item				2	3
Total repurchases		$ 200.0		$ 450.0
Cash paid for repurchase of common stock		$ 200.0
Shares repurchased | shares	0.4	1.8		5.4	7.2
Weighted average per share purchase price | $ / shares	$ 90.07				$ 85.04
10b5-1 Plan [Member]
Accelerated Share Repurchases [Line Items]
Total repurchases			$ 100.0
Shares repurchased | shares			1.2
Weighted average per share purchase price | $ / shares			$ 83.20