Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	$ 3,632
Available-for-sale securities, Debt instruments, Fair value	3,628
Amortized cost	5,734	$ 6,273
Unrealized gains	46	285
Unrealized losses	(41)	(18)
Fair value	5,739	6,540
U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	2,721	2,833
Unrealized gains	1	3
Unrealized losses	(7)	(1)
Available-for-sale securities, Debt instruments, Fair value	2,715	2,835
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	9	10
Unrealized gains	1	1
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	10	11
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	405	500
Unrealized gains	0	0
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	405	500
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	384	482
Unrealized gains	2	7
Unrealized losses	(1)	(2)
Available-for-sale securities, Debt instruments, Fair value	385	487
U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	47	57
Unrealized gains	1	2
Unrealized losses	(1)	0
Available-for-sale securities, Debt instruments, Fair value	47	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	2
Unrealized gains	0	1
Unrealized losses	0	0
Available-for-sale securities, Debt instruments, Fair value	2	3
Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	34	42
Unrealized gains	1	1
Unrealized losses	(1)	(1)
Available-for-sale securities, Debt instruments, Fair value	34	42
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	30	38
Unrealized gains	0	0
Unrealized losses	0	(1)
Available-for-sale securities, Debt instruments, Fair value	30	37
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	39	178
Unrealized gains	0	0
Unrealized losses	0	0
Equity instrument, Fair value	39	178
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	396	353
Unrealized gains	0	6
Unrealized losses	(17)	(1)
Equity instrument, Fair value	379	358
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	21	19
Unrealized gains	0	0
Unrealized losses	(1)	0
Equity instrument, Fair value	20	19
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	254	243
Unrealized gains	8	8
Unrealized losses	(5)	(5)
Equity instrument, Fair value	257	246
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	83	67
Unrealized gains	5	0
Unrealized losses	0	(1)
Equity instrument, Fair value	88	66
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	147	201
Unrealized gains	5	11
Unrealized losses	0	0
Equity instrument, Fair value	152	212
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	393	297
Unrealized gains	0	5
Unrealized losses	(6)	0
Equity instrument, Fair value	387	302
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	141	117
Unrealized gains	8	8
Unrealized losses	(1)	(1)
Equity instrument, Fair value	148	124
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	106	101
Unrealized gains	8	1
Unrealized losses	0	0
Equity instrument, Fair value	114	102
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	136	145
Unrealized gains	3	0
Unrealized losses	0	(5)
Equity instrument, Fair value	139	140
Fixed income total return bond funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	386	376
Unrealized gains	3	4
Unrealized losses	(1)	0
Equity instrument, Fair value	$ 388	380
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost		50
Unrealized gains		0
Unrealized losses		0
Available-for-sale securities, Debt instruments, Fair value		50
Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost		162
Unrealized gains		227
Unrealized losses		0
Equity instrument, Fair value		$ 389

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