Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	$ 633
Equity investments	2,275	$ 2,184
Investments in marketable securities total	2,908	5,829
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 securities	212	2,798
Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	11	11
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	162	201
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 securities	35	39
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 securities	1	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 securities	39	29
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	53	39
Recurring [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	633
Equity investments	2,275
Investments in marketable securities total	2,908
Derivative assets	61	61
Counterparty netting and collateral	(483)	(1,297)
Assets at fair value	2,969	5,890
Counterparty netting and collateral	1,381	822
Liabilities at fair value	(26)	(6)
Net assets at fair value	2,943	5,884
Derivative asset liability collateral obligation right to return reclaim cash offset	898	(475)
Available-for-sale securities total		5,829
Recurring [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	50	474
Recurring [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	70	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 securities	212	2,798
Recurring [Member] | Municipal debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	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 securities	162	201
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 securities	35	39
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 securities	1	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 securities	39	29
Recurring [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	53	39
Recurring [Member] | Fixed income total return bond funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Equity investments	1,586
Available-for-sale securities, Equity investments		1,524
Recurring [Member] | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	72	485
Derivative liabilities	(785)	(200)
Recurring [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	472	872
Derivative liabilities	(622)	(628)
Recurring [Member] | Interest rate floors [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets		1
Recurring [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	194
Equity investments	1,586
Investments in marketable securities total	1,780
Derivative assets	0	0
Counterparty netting and collateral	0	0
Assets at fair value	1,780	4,313
Counterparty netting and collateral	0	0
Liabilities at fair value	0	0
Net assets at fair value	1,780	4,313
Available-for-sale securities total		4,313
Recurring [Member] | Level 1 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	0	0
Recurring [Member] | Level 1 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	0	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 securities	194	2,774
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 securities	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 securities	0	15
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 securities	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 securities	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 securities	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 securities	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]
Equity investments	1,586
Available-for-sale securities, Equity investments		1,524
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
Recurring [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	346
Equity investments	0
Investments in marketable securities total	346
Derivative assets	543	1,358
Counterparty netting and collateral	0	0
Assets at fair value	889	2,144
Counterparty netting and collateral	0	0
Liabilities at fair value	(1,407)	(807)
Net assets at fair value	(518)	1,337
Available-for-sale securities total		786
Recurring [Member] | Level 2 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	50	474
Recurring [Member] | Level 2 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	70	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 securities	18	24
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 securities	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 securities	162	186
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 securities	35	39
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 securities	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 securities	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 securities	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]
Equity investments	0
Available-for-sale securities, Equity investments		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	72	485
Derivative liabilities	(785)	(200)
Recurring [Member] | Level 2 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	471	872
Derivative liabilities	(622)	(607)
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
Recurring [Member] | Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	93
Equity investments	0
Investments in marketable securities total	93
Derivative assets	1	0
Counterparty netting and collateral	0	0
Assets at fair value	94	70
Counterparty netting and collateral	0	0
Liabilities at fair value	0	(21)
Net assets at fair value	94	49
Available-for-sale securities total		70
Recurring [Member] | Level 3 [Member] | Certificates of deposit [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	0	0
Recurring [Member] | Level 3 [Member] | Commercial paper [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Available-for-sale securities, Debt securities	0	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 securities	0	0
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 securities	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 securities	0	0
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 securities	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 securities	1	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 securities	39	29
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 securities	53	39
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]
Equity investments	0
Available-for-sale securities, Equity investments		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	0
Recurring [Member] | Level 3 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative assets	1	0
Derivative liabilities	0	(21)
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
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]
Equity investments	$ 689
Available-for-sale securities, Equity investments		$ 660