Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 91		$ 79
Gains/(losses) included in earnings	28		19
Gains/(losses) included in other comprehensive income	1		1
Purchases	3		4
Issuances	0		0
Sales	(3)		(4)
Settlements	(13)		(19)
Transfers into Level 3	0		27
Transfers out of Level 3	0		0
Fair value,	107		107
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	22		13
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	48		21
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	1		1
Purchases	3		4
Issuances	0		0
Sales	(3)		(4)
Settlements	(5)		(5)
Transfers into Level 3	0		27
Transfers out of Level 3	0		0
Fair value,	44		44
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	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,	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,	25		15
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	1		1
Purchases	2		3
Issuances	0		0
Sales	(2)		(3)
Settlements	(2)		(2)
Transfers into Level 3	0		10
Transfers out of Level 3	0		0
Fair value,	24		24
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	12		1
Gains/(losses) included in earnings	0		0
Gains/(losses) included in other comprehensive income	0		0
Purchases	1		1
Issuances	0		0
Sales	(1)		(1)
Settlements	(3)		(3)
Transfers into Level 3	0		11
Transfers out of Level 3	0		0
Fair value,	9		9
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	43	122	58	75
Gains/(losses) included in earnings	28	25	19	88
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	(8)	(2)	(14)	(18)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	63	145	63	145
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	22	24	13	69
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	11	10	13	17
Gains/(losses) included in earnings	2	2	1	6
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)	(2)	(2)	(13)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	1	0	1
Fair value,	12	11	12	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	2	1	1	3
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	59	152	69	109
Gains/(losses) included in earnings	22	28	17	76
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	(7)	0	(12)	(5)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	74	180	74	180
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	22	28	16	76
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	(27)	(40)	(24)	(51)
Gains/(losses) included in earnings	4	(5)	1	6
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	(1)	0	(1)
Fair value,	(23)	(46)	(23)	(46)
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)	$ (5)	$ (4)	$ (10)