Investments - Available-For-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less (including restricted cash of $19)	$ 2,279
One year through two years	2,044
Two years through three years	1,723
Three years through four years	735
Four years through five years	515
Five years through six years	0
Estimated Fair Value	7,296
Restricted cash	19
Short-term Investments
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	2,283	$ 3,746
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(5)	(4)
Estimated Fair Value	2,279	3,743
Short-term Investments | Restricted cash
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	19	20
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	19	20
Short-term Investments | Corporate debt securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	2,264	3,726
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(5)	(4)
Estimated Fair Value	2,260	3,723
Long-term Investments
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	5,064	5,458
Gross Unrealized Gains	7	12
Gross Unrealized Losses	(54)	(24)
Estimated Fair Value	5,017	5,446
Long-term Investments | Corporate debt securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	5,064	5,458
Gross Unrealized Gains	7	12
Gross Unrealized Losses	(54)	(24)
Estimated Fair Value	$ 5,017	$ 5,446