Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	$ 4,508
Available-for-sale securities, Debt instruments, Fair value	4,507
Amortized cost	6,652	$ 5,652
Unrealized gains	35	50
Unrealized losses	(9)	(10)
Fair value	6,678	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	1,795	2,314
Unrealized gains	1	1
Unrealized losses	(7)	(7)
Available-for-sale securities, Debt instruments, Fair value	1,789	2,308
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	10	10
Unrealized gains	1	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	2,200	755
Unrealized gains	0	0
Unrealized losses	(1)	0
Available-for-sale securities, Debt instruments, Fair value	2,199	755
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	382	368
Unrealized gains	4	2
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	386	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	43	45
Unrealized gains	1	1
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	44	45
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	2	2
Unrealized gains	0	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	41	37
Unrealized gains	0	1
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	41	37
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	35	31
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	35	31
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	24	39
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	24	39
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	161	389
Unrealized gains	4	0
Unrealized losses	0	0
Equity instrument, Fair value	165	389
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	7	20
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	7	20
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	94	252
Unrealized gains	1	9
Unrealized losses	0	0
Equity instrument, Fair value	95	261
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	30	83
Unrealized gains	2	6
Unrealized losses	0	0
Equity instrument, Fair value	32	89
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	36	147
Unrealized gains	1	5
Unrealized losses	0	0
Equity instrument, Fair value	37	152
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	145	390
Unrealized gains	2	2
Unrealized losses	0	0
Equity instrument, Fair value	147	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	1	10
Unrealized losses	0	0
Equity instrument, Fair value	57	150
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	38	105
Unrealized gains	3	7
Unrealized losses	0	0
Equity instrument, Fair value	41	112
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	54	136
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	54	136
Fixed income total return bond funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	1,499	389
Unrealized gains	14	5
Unrealized losses	(1)	0
Equity instrument, Fair value	$ 1,512	$ 394

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