Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 79	$ 75
Gains/(losses) included in earnings	25	86
Gains/(losses) included in other comprehensive income	0	0
Purchases	44	0
Issuances	0	0
Sales	(7)	0
Settlements	(40)	(46)
Transfers into Level 3	27	21
Transfers out of Level 3	0	(57)
Fair value,	128	79
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or losses related to assets still held at the reporting date	11	46
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	21	0
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	44	0
Issuances	0	0
Sales	(7)	0
Settlements	(12)	0
Transfers into Level 3	27	21
Transfers out of Level 3	0	0
Fair value,	73	21
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	1	0
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	3	1
Transfers out of Level 3	0	0
Fair value,	4	1
Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	4	0
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	2	0
Issuances	0	0
Sales	(2)	0
Settlements	(2)	0
Transfers into Level 3	3	4
Transfers out of Level 3	0	0
Fair value,	5	4
Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	15	0
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	35	0
Issuances	0	0
Sales	(4)	0
Settlements	(5)	0
Transfers into Level 3	10	15
Transfers out of Level 3	0	0
Fair value,	51	15
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	1	0
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	7	0
Issuances	0	0
Sales	(1)	0
Settlements	(5)	0
Transfers into Level 3	11	1
Transfers out of Level 3	0	0
Fair value,	13	1
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	58	75
Gains/(losses) included in earnings	25	86
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(28)	(46)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(57)
Fair value,	55	58
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or losses related to assets still held at the reporting date	11	46
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	13	17
Gains/(losses) included in earnings	2	11
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(3)	(16)
Transfers into Level 3	0	0
Transfers out of Level 3	0	1
Fair value,	12	13
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or losses related to assets still held at the reporting date	2	7
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	69	109
Gains/(losses) included in earnings	11	47
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(25)	(30)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(57)
Fair value,	55	69
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or losses related to assets still held at the reporting date	8	43
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	(24)	(51)
Gains/(losses) included in earnings	12	28
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	(1)
Fair value,	(12)	(24)
The amount of total gains or (losses) for the period included in earnings attributable to the change in unrealized gains or losses related to assets still held at the reporting date	$ 1	$ (4)