Investments in Marketable Securities - Summary of Investments in Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 6,775	$ 5,065
Available-for-sale securities, Debt instruments, Amortized cost	4,859
Unrealized gains	366	333
Unrealized losses	(10)	(9)
Available-for-sale securities, Debt instruments, Fair value	4,874
Equity instrument, Amortized cost	0	0
Equity instrument, Fair value	0	0
Fair value	7,131	5,389
Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	0	0
U.S. government and agency obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	4,357	652
Available-for-sale securities, Debt instruments, Fair value	4,359	652
U.S. government and agency obligations [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	3	1
Unrealized losses	(1)	(1)
Municipal debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	10	10
Available-for-sale securities, Debt instruments, Fair value	12	11
Municipal debt securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	2	1
Unrealized losses	0	0
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	175	1,599
Available-for-sale securities, Debt instruments, Fair value	175	1,599
Certificates of deposit [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	0	0
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	37	507
Available-for-sale securities, Debt instruments, Fair value	37	507
Commercial paper [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	0	0
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	138	164
Available-for-sale securities, Debt instruments, Fair value	145	169
Corporate debt securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	7	6
Unrealized losses	0	(1)
U.S. government agency mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	57	60
Available-for-sale securities, Debt instruments, Fair value	59	60
U.S. government agency mortgage-backed securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	2	1
Unrealized losses	0	(1)
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	3	4
Available-for-sale securities, Debt instruments, Fair value	4	5
Non-agency residential mortgage-backed securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	1	1
Unrealized losses	0	0
Non-agency commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	43	44
Available-for-sale securities, Debt instruments, Fair value	44	43
Non-agency commercial mortgage-backed securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	1	1
Unrealized losses	0	(2)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Debt instruments, Amortized cost	39	27
Available-for-sale securities, Debt instruments, Fair value	39	27
Asset-backed securities [Member] | Debt instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	0	0
Fixed Income Funds
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	0	0
Equity instrument, Fair value	0	0
Fixed Income Funds | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	0	0
Fixed income short-term floating NAV fund II [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	148
Equity instrument, Fair value	148
Fixed income short-term floating NAV fund II [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0
Unrealized losses	0
Fixed income short-term sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	35	41
Equity instrument, Fair value	37	44
Fixed income short-term sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	2	3
Unrealized losses	0	0
Fixed income U.S. government sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	311	329
Equity instrument, Fair value	335	327
Fixed income U.S. government sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	24	0
Unrealized losses	0	(2)
Fixed income municipal sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	19	21
Equity instrument, Fair value	20	22
Fixed income municipal sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	1	1
Unrealized losses	0	0
Investment Grade Corporate Sector Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	256	283
Equity instrument, Fair value	268	316
Investment Grade Corporate Sector Fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	15	33
Unrealized losses	(3)	0
Fixed income high-yield sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	50	38
Equity instrument, Fair value	55	45
Fixed income high-yield sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	6	7
Unrealized losses	(1)	0
Fixed income real return sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	235	275
Equity instrument, Fair value	232	274
Fixed income real return sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	0
Unrealized losses	(3)	(1)
Fixed income mortgage sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	390	519
Equity instrument, Fair value	399	520
Fixed income mortgage sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	9	1
Unrealized losses	0	0
Fixed income asset-backed securities sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	63	40
Equity instrument, Fair value	72	50
Fixed income asset-backed securities sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	9	10
Unrealized losses	0	0
Fixed income emerging market sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	73	65
Equity instrument, Fair value	71	66
Fixed income emerging market sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	0	1
Unrealized losses	(2)	0
Fixed income international sector fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	146	170
Equity instrument, Fair value	160	171
Fixed income international sector fund [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	14	2
Unrealized losses	0	(1)
Equity mutual fund - S&P 500 index [Member]
Schedule of Available-for-sale Securities [Line Items]
Equity instrument, Amortized cost	190	217
Equity instrument, Fair value	460	481
Equity mutual fund - S&P 500 index [Member] | Equity instruments [Member]
Schedule of Available-for-sale Securities [Line Items]
Unrealized gains	270	264
Unrealized losses	$ 0	$ 0