Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Sep. 30, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 6,153	$ 6,775
Available-for-sale securities, Debt instruments, Amortized cost	4,250
Available-for-sale securities, Debt instruments, Fair value	4,257
Unrealized gains	272	366
Unrealized losses	(15)	(10)
Fair value	6,410	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	3,354	4,357
Unrealized gains	4	3
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	3,358	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	400	175
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	400	175
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	124	37
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	124	37
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	231	138
Unrealized gains	3	7
Unrealized losses	(3)	0
Available-for-sale securities, Debt instruments, Fair value	231	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	51	57
Unrealized gains	1	2
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	52	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	41	43
Unrealized gains	1	1
Unrealized losses	(1)	0
Available-for-sale securities, Debt instruments, Fair value	41	44
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	36	39
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	36	39
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	53	148
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	53	148
Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost		35
Unrealized gains		2
Unrealized losses		0
Equity instrument, Fair value		37
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	317	311
Unrealized gains	7	24
Unrealized losses	0	0
Equity instrument, Fair value	324	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	253	256
Unrealized gains	11	15
Unrealized losses	0	(3)
Equity instrument, Fair value	264	268
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	97	50
Unrealized gains	4	6
Unrealized losses	0	(1)
Equity instrument, Fair value	101	55
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	225	235
Unrealized gains	0	0
Unrealized losses	0	(3)
Equity instrument, Fair value	225	232
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	420	390
Unrealized gains	10	9
Unrealized losses	0	0
Equity instrument, Fair value	430	399
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	64	63
Unrealized gains	9	9
Unrealized losses	0	0
Equity instrument, Fair value	73	72
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	119	73
Unrealized gains	0	0
Unrealized losses	0	(2)
Equity instrument, Fair value	119	71
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	161	146
Unrealized gains	0	14
Unrealized losses	(11)	0
Equity instrument, Fair value	150	160
Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	175	190
Unrealized gains	219	270
Unrealized losses	0	0
Equity instrument, Fair value	$ 394	$ 460

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