Subsequent Event (Subsequent Event - Additional Information) (Detail) $ in Millions	3 Months Ended
	Jun. 30, 2018 USD ($) shares
Subsequent Event
Subsequent Event [Line Items]
Subsequent Event Date	Jul. 26, 2018
Face amount of notes issued or redeemed	$ 1,550
Fixed Rate 3.35 Percent Senior Notes Due 2021
Subsequent Event [Line Items]
Face amount of notes issued or redeemed	$ 500
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 26, 2021
Floating Rate 3ML Plus 44 bps Senior Notes Due 2021
Subsequent Event [Line Items]
Face amount of notes issued or redeemed	$ 300
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 26, 2021
Fixed Rate 3.95 Percent Senior Notes Due 2025
Subsequent Event [Line Items]
Face amount of notes issued or redeemed	$ 750
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 28, 2025
Open Market Share Repurchase
Subsequent Event [Line Items]
Subsequent Event Start Date	Jul. 20, 2018
Subsequent Event End Date	Aug. 02, 2018
Shares repurchased | shares	16,945,020
Repurchased common stock through open market repurchase transactions	$ 500
Settlement Start Date	Jul. 24, 2018
Settlement End Date	Aug. 06, 2018