Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Mar. 31, 2016
Short-term Debt [Line Items]
Tax benefit recognized related to excess tax benefits arising from adoption of ASU 2016-09		$ 25
Effect of Early Adoption of ASU 2016-09 [Member]
Short-term Debt [Line Items]
Tax benefit recognized related to excess tax benefits arising from adoption of ASU 2016-09	$ 9		$ 46
Adjustments for New Accounting Pronouncement [Member] | Other Noncurrent Assets
Short-term Debt [Line Items]
Unamortized debt issuance costs				$ (40)
Adjustments for New Accounting Pronouncement [Member] | Long-term Debt
Short-term Debt [Line Items]
Unamortized debt issuance costs				$ 40