Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis Using Significant Unobservable Inputs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	$ 162	$ 128
(Losses)/gains included in earnings	(54)	34
(Losses)/gains included in other comprehensive income	2	(2)
Purchases	37	26
Issuances	0	0
Sales	(15)	(8)
Settlements	(23)	(23)
Transfers in to Level 3	2	9
Transfers out of Level 3	0	(2)
Fair value	111	162
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	(54)	21
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	89	73
(Losses)/gains included in earnings	0	0
(Losses)/gains included in other comprehensive income	2	(2)
Purchases	37	26
Issuances	0	0
Sales	(15)	(8)
Settlements	(12)	(7)
Transfers in to Level 3	2	9
Transfers out of Level 3	0	(2)
Fair value	103	89
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	0
Available-for-sale securities [Member] | U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	2	0
(Losses)/gains included in earnings	0	0
(Losses)/gains included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Transfers in to Level 3	0	2
Transfers out of Level 3	0	0
Fair value	2	2
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	0
Available-for-sale securities [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	12	4
(Losses)/gains included in earnings	0	0
(Losses)/gains included in other comprehensive income	0	0
Purchases	3	3
Issuances	0	0
Sales	(3)	0
Settlements	0	0
Transfers in to Level 3	2	7
Transfers out of Level 3	0	(2)
Fair value	14	12
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	0
Available-for-sale securities [Member] | Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	48	56
(Losses)/gains included in earnings	0	0
(Losses)/gains included in other comprehensive income	2	(2)
Purchases	12	7
Issuances	0	0
Sales	(7)	(8)
Settlements	(7)	(5)
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value	48	48
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	0
Available-for-sale securities [Member] | Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	27	13
(Losses)/gains included in earnings	0	0
(Losses)/gains included in other comprehensive income	0	0
Purchases	22	16
Issuances	0	0
Sales	(5)	0
Settlements	(5)	(2)
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value	39	27
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	0
Derivative
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	73	55
(Losses)/gains included in earnings	(54)	34
(Losses)/gains included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(11)	(16)
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value	8	73
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	(54)	21
Derivative | Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	3	12
(Losses)/gains included in earnings	0	5
(Losses)/gains included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(2)	(14)
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value	1	3
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	0	(2)
Derivative | Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value	70	55
(Losses)/gains included in earnings	(54)	17
(Losses)/gains included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(9)	(2)
Transfers in to Level 3	0	0
Transfers out of Level 3	0	0
Fair value	7	70
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date	(54)	23
Derivative | Embedded Derivative Financial Instruments
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value		(12)
(Losses)/gains included in earnings		12
(Losses)/gains included in other comprehensive income		0
Purchases		0
Issuances		0
Sales		0
Settlements		0
Transfers in to Level 3		0
Transfers out of Level 3		0
Fair value		0
The amount of total (losses) gains included in earnings attributable to assets still held at the reporting date		$ 0