Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	$ 3,645
Available-for-sale securities total	5,829	$ 5,692
Derivative assets	61	51
Counterparty netting and collateral	(737)	(154)
Derivative liabilities	(6)	(46)
Counterparty netting and collateral	262	1,013
U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2,798	2,308
Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	11	11
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	201	369
U.S. government agency mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	45
Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2	2
Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	29	37
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	31
Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	1,524	394
Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	3,392	3,962
Available-for-sale securities total	5,829	5,692
Derivative assets	61	51
Counterparty netting and collateral	(1,297)	(548)
Assets at fair value	10,164	9,705
Counterparty netting and collateral	822	1,407
Liabilities at fair value	(6)	(46)
Net assets at fair value	10,158	9,659
Derivative asset liability collateral obligation right to return reclaim cash offset	(475)	859
Recurring [Member] | Money market instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	897	1,332
Restricted cash equivalents	882
Recurring [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	2,240	2,630
Available-for-sale securities, Debt instruments	474	755
Recurring [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	255
Available-for-sale securities, Debt instruments	52
Recurring [Member] | U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2,798	2,308
Recurring [Member] | Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	11	11
Recurring [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	201	369
Recurring [Member] | U.S. government agency mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	45
Recurring [Member] | Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2	2
Recurring [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	29	37
Recurring [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	31
Recurring [Member] | Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	1,524	394
Recurring [Member] | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	485	122
Derivative liabilities	(200)	(1,176)
Recurring [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	872	475
Derivative liabilities	(628)	(277)
Recurring [Member] | Interest rate floors [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	1	2
Recurring [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	347	287
Available-for-sale securities total	4,313	2,983
Derivative assets	0	0
Counterparty netting and collateral	0	0
Assets at fair value	4,833	3,270
Counterparty netting and collateral	0	0
Liabilities at fair value	0	0
Net assets at fair value	4,833	3,270
Recurring [Member] | Level 1 [Member] | Money market instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	347	287
Restricted cash equivalents	173
Recurring [Member] | Level 1 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Available-for-sale securities, Debt instruments	0	105
Recurring [Member] | Level 1 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Available-for-sale securities, Debt instruments	0
Recurring [Member] | Level 1 [Member] | U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2,774	2,273
Recurring [Member] | Level 1 [Member] | Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 1 [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	15	211
Recurring [Member] | Level 1 [Member] | U.S. government agency mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 1 [Member] | Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 1 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 1 [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 1 [Member] | Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	1,524	394
Recurring [Member] | Level 1 [Member] | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Recurring [Member] | Level 1 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Recurring [Member] | Level 1 [Member] | Interest rate floors [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	0
Recurring [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	3,045	3,675
Available-for-sale securities total	786	889
Derivative assets	1,358	598
Counterparty netting and collateral	0	0
Assets at fair value	5,898	5,162
Counterparty netting and collateral	0	0
Liabilities at fair value	(807)	(1,385)
Net assets at fair value	5,091	3,777
Recurring [Member] | Level 2 [Member] | Money market instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	550	1,045
Restricted cash equivalents	709
Recurring [Member] | Level 2 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	2,240	2,630
Available-for-sale securities, Debt instruments	474	650
Recurring [Member] | Level 2 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	255
Available-for-sale securities, Debt instruments	52
Recurring [Member] | Level 2 [Member] | U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	24	33
Recurring [Member] | Level 2 [Member] | Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	11	11
Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	186	150
Recurring [Member] | Level 2 [Member] | U.S. government agency mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	45
Recurring [Member] | Level 2 [Member] | Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 2 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 2 [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 2 [Member] | Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	0	0
Recurring [Member] | Level 2 [Member] | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	485	122
Derivative liabilities	(200)	(1,114)
Recurring [Member] | Level 2 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	872	474
Derivative liabilities	(607)	(271)
Recurring [Member] | Level 2 [Member] | Interest rate floors [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	1	2
Recurring [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Available-for-sale securities total	70	80
Derivative assets	0	1
Counterparty netting and collateral	0	0
Assets at fair value	70	81
Counterparty netting and collateral	0	0
Liabilities at fair value	(21)	(68)
Net assets at fair value	49	13
Recurring [Member] | Level 3 [Member] | Money market instruments [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash equivalents	0
Recurring [Member] | Level 3 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 3 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cash equivalents	0
Available-for-sale securities, Debt instruments	0
Recurring [Member] | Level 3 [Member] | U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	2
Recurring [Member] | Level 3 [Member] | Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 3 [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	8
Recurring [Member] | Level 3 [Member] | U.S. government agency mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	0	0
Recurring [Member] | Level 3 [Member] | Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	2	2
Recurring [Member] | Level 3 [Member] | Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	29	37
Recurring [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt instruments	39	31
Recurring [Member] | Level 3 [Member] | Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	0	0
Recurring [Member] | Level 3 [Member] | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	(62)
Recurring [Member] | Level 3 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	1
Derivative liabilities	(21)	(6)
Recurring [Member] | Level 3 [Member] | Interest rate floors [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	0	0
Recurring [Member] | Fixed income mutual funds measured at net asset value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Equity instruments	$ 660	$ 1,740