Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 8,020	$ 6,775
Available-for-sale securities, Debt instruments, Amortized cost	6,135
Available-for-sale securities, Debt instruments, Fair value	6,128
Unrealized gains	262	366
Unrealized losses	(41)	(10)
Fair value	8,241	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	4,126	4,357
Unrealized gains	1	3
Unrealized losses	(9)	(1)
Available-for-sale securities, Debt instruments, Fair value	4,118	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	1,325	175
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	1,325	175
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	175	37
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	175	37
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	369	138
Unrealized gains	3	7
Unrealized losses	(4)	0
Available-for-sale securities, Debt instruments, Fair value	368	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	49	57
Unrealized gains	1	2
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	50	59
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	2	3
Unrealized gains	1	1
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	3	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	(1)	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	180	148
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	180	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	259	311
Unrealized gains	0	24
Unrealized losses	(4)	0
Equity instrument, Fair value	255	335
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	19	19
Unrealized gains	0	1
Unrealized losses	0	0
Equity instrument, Fair value	19	20
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	243	256
Unrealized gains	8	15
Unrealized losses	(7)	(3)
Equity instrument, Fair value	244	268
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	97	50
Unrealized gains	5	6
Unrealized losses	(1)	(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	(4)	(3)
Equity instrument, Fair value	221	232
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	330	390
Unrealized gains	0	9
Unrealized losses	(3)	0
Equity instrument, Fair value	327	399
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	117	63
Unrealized gains	8	9
Unrealized losses	(1)	0
Equity instrument, Fair value	124	72
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	101	73
Unrealized gains	0	0
Unrealized losses	(6)	(2)
Equity instrument, Fair value	95	71
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	145	146
Unrealized gains	0	14
Unrealized losses	0	0
Equity instrument, Fair value	145	160
Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	169	190
Unrealized gains	233	270
Unrealized losses	0	0
Equity instrument, Fair value	$ 402	$ 460

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