Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	$ 3,928
Available-for-sale securities, Debt instruments, Fair value	3,928
Amortized cost	6,089	$ 5,652
Unrealized gains	49	50
Unrealized losses	(10)	(10)
Fair value	6,128	5,692
U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	2,576	2,314
Unrealized gains	0	1
Unrealized losses	(8)	(7)
Available-for-sale securities, Debt instruments, Fair value	2,568	2,308
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	9	10
Unrealized gains	2	1
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	11	11
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	875	755
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	875	755
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	5
Unrealized gains	0
Unrealized losses	0
Available-for-sale securities, Debt instruments, Fair value	5
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	347	368
Unrealized gains	5	2
Unrealized losses	(1)	(1)
Available-for-sale securities, Debt instruments, Fair value	351	369
U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	41	45
Unrealized gains	1	1
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	42	45
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	1	2
Unrealized gains	1	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	2	2
Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	38	37
Unrealized gains	0	1
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	38	37
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	36	31
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	36	31
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	30	39
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	30	39
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	189	389
Unrealized gains	5	0
Unrealized losses	(1)	0
Equity instrument, Fair value	193	389
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	8	20
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	8	20
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	95	252
Unrealized gains	1	9
Unrealized losses	0	0
Equity instrument, Fair value	96	261
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	30	83
Unrealized gains	3	6
Unrealized losses	0	0
Equity instrument, Fair value	33	89
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	5	147
Unrealized gains	0	5
Unrealized losses	0	0
Equity instrument, Fair value	5	152
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	146	390
Unrealized gains	3	2
Unrealized losses	0	0
Equity instrument, Fair value	149	392
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	56	140
Unrealized gains	2	10
Unrealized losses	0	0
Equity instrument, Fair value	58	150
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	38	105
Unrealized gains	4	7
Unrealized losses	0	0
Equity instrument, Fair value	42	112
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	54	136
Unrealized gains	1	0
Unrealized losses	0	0
Equity instrument, Fair value	55	136
Fixed income total return bond funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	1,510	389
Unrealized gains	21	5
Unrealized losses	0	0
Equity instrument, Fair value	$ 1,531	$ 394

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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