Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 107		$ 79
Gains/(losses) included in earnings	16		35
Gains/(losses) included in other comprehensive income	0		1
Purchases	2		6
Issuances	0		0
Sales	0		(4)
Settlements	(17)		(36)
Transfers into Level 3	0		27
Transfers out of Level 3	0		0
Fair value,	108		108
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	10		23
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	44		21
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	0		1
Purchases	2		6
Issuances	0		0
Sales	0		(4)
Settlements	(4)		(9)
Transfers into Level 3	0		27
Transfers out of Level 3	0		0
Fair value,	42		42
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	4		1
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	0		3
Transfers out of Level 3	0		0
Fair value,	4		4
Non-agency residential mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	7		4
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	0		0
Purchases	2		2
Issuances	0		0
Sales	0		0
Settlements	(2)		(2)
Transfers into Level 3	0		3
Transfers out of Level 3	0		0
Fair value,	7		7
Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	24		15
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	0		1
Purchases	0		3
Issuances	0		0
Sales	0		(3)
Settlements	(1)		(3)
Transfers into Level 3	0		10
Transfers out of Level 3	0		0
Fair value,	23		23
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	9		1
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	0		0
Purchases	0		1
Issuances	0		0
Sales	0		(1)
Settlements	(1)		(4)
Transfers into Level 3	0		11
Transfers out of Level 3	0		0
Fair value,	8		8
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	63	145	58	75
Gains/(losses) included in earnings	16	17	35	106
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(13)	(13)	(27)	(32)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	66	149	66	149
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	10	9	23	80
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	12	11	13	17
Gains/(losses) included in earnings	0	1	1	8
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(1)	(1)	(3)	(15)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	1
Fair value,	11	11	11	11
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	0	1	1	5
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	74	180	69	109
Gains/(losses) included in earnings	6	13	23	89
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(12)	(12)	(24)	(17)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	68	181	68	181
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	6	13	21	88
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	(23)	(46)	(24)	(51)
Gains/(losses) included in earnings	10	3	11	9
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(1)
Fair value,	(13)	(43)	(13)	(43)
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	$ 4	$ (5)	$ 1	$ (13)