Long-Term Debt - Unsecured Debt Offerings Narrative (Details) - Unsecured Debt - USD ($)	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Debt instrument, face amount	$ 450,000,000	$ 2,000,000,000.0
2.875% Notes due 2020
Debt Instrument [Line Items]
Debt instrument, face amount		$ 1,000,000,000.0
Debt instrument, stated interest rate (percent)		2.875%
3.625% Notes due 2022
Debt Instrument [Line Items]
Debt instrument, face amount		$ 1,000,000,000.0
Debt instrument, stated interest rate (percent)		3.625%
2.600% Notes due 2020
Debt Instrument [Line Items]
Debt instrument, stated interest rate (percent)	2.60%