Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 6,297	$ 6,775
Available-for-sale securities, Debt instruments, Amortized cost	4,373
Available-for-sale securities, Debt instruments, Fair value	4,380
Unrealized gains	313	366
Unrealized losses	(25)	(10)
Fair value	6,585	7,131
U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	3,936	4,357
Unrealized gains	2	3
Unrealized losses	(1)	(1)
Available-for-sale securities, Debt instruments, Fair value	3,937	4,359
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	2
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	11	12
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	100	175
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	100	175
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	37	37
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	37	37
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	155	138
Unrealized gains	4	7
Unrealized losses	(2)	0
Available-for-sale securities, Debt instruments, Fair value	157	145
U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	53	57
Unrealized gains	2	2
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	55	59
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	3	3
Unrealized gains	1	1
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	4	4
Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	42	43
Unrealized gains	1	1
Unrealized losses	(1)	0
Available-for-sale securities, Debt instruments, Fair value	42	44
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	37	39
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	37	39
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	122	148
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	122	148
Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	36	35
Unrealized gains	2	2
Unrealized losses	0	0
Equity instrument, Fair value	38	37
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	312	311
Unrealized gains	4	24
Unrealized losses	(3)	0
Equity instrument, Fair value	313	335
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	19	19
Unrealized gains	1	1
Unrealized losses	0	0
Equity instrument, Fair value	20	20
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	257	256
Unrealized gains	14	15
Unrealized losses	(4)	(3)
Equity instrument, Fair value	267	268
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	51	50
Unrealized gains	6	6
Unrealized losses	(1)	(1)
Equity instrument, Fair value	56	55
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	236	235
Unrealized gains	0	0
Unrealized losses	(8)	(3)
Equity instrument, Fair value	228	232
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	418	390
Unrealized gains	6	9
Unrealized losses	(1)	0
Equity instrument, Fair value	423	399
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	63	63
Unrealized gains	9	9
Unrealized losses	0	0
Equity instrument, Fair value	72	72
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	74	73
Unrealized gains	0	0
Unrealized losses	(2)	(2)
Equity instrument, Fair value	72	71
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	153	146
Unrealized gains	0	14
Unrealized losses	(2)	0
Equity instrument, Fair value	151	160
Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	183	190
Unrealized gains	260	270
Unrealized losses	0	0
Equity instrument, Fair value	$ 443	$ 460

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