Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 103	$ 91	$ 128	$ 79
Gains/(losses) included in earnings	28	28	12	19
Gains/(losses) included in other comprehensive income	0	1	(3)	1
Purchases	2	3	9	4
Issuances	0	0	0	0
Sales	0	(3)	0	(4)
Settlements	(2)	(13)	(15)	(19)
Transfers into Level 3	2	0	2	27
Transfers out of Level 3	(2)	0	(2)	0
Fair value,	131	107	131	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	28	22	8	13
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	76	48	73	21
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	1	(3)	1
Purchases	2	3	9	4
Issuances	0	0	0	0
Sales	0	(3)	0	(4)
Settlements	(1)	(5)	(2)	(5)
Transfers into Level 3	2	0	2	27
Transfers out of Level 3	(2)	0	(2)	0
Fair value,	77	44	77	44
U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	0		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	2		2
Transfers out of Level 3	0		0
Fair value,	2		2
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	4	4	4	1
Gains/(losses) included in earnings	0	0	0	0
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	3
Transfers out of Level 3	(2)	0	(2)	0
Fair value,	2	4	2	4
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	53	32	56	19
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	1	(3)	1
Purchases	2	2	2	3
Issuances	0	0	0	0
Sales	0	(2)	0	(3)
Settlements	(1)	(2)	(1)	(2)
Transfers into Level 3	0	0	0	13
Transfers out of Level 3	0	0	0	0
Fair value,	54	31	54	31
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	19	12	13	1
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	0	1	7	1
Issuances	0	0	0	0
Sales	0	(1)	0	(1)
Settlements	0	(3)	(1)	(3)
Transfers into Level 3	0	0	0	11
Transfers out of Level 3	0	0	0	0
Fair value,	19	9	19	9
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	27	43	55	58
Gains/(losses) included in earnings	28	28	12	19
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)	(8)	(13)	(14)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	54	63	54	63
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	28	22	8	13
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	11	11	12	13
Gains/(losses) included in earnings	2	2	2	1
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	(1)	(1)	(2)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	13	12	13	12
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	2	2	1
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	23	59	55	69
Gains/(losses) included in earnings	28	22	7	17
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)	(7)	(12)	(12)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	50	74	50	74
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	28	22	9	16
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	(7)	(27)	(12)	(24)
Gains/(losses) included in earnings	(2)	4	3	1
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	0
Fair value,	(9)	(23)	(9)	(23)
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)	$ (2)	$ (3)	$ (4)