Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 169	$ 103	$ 162	$ 128
Gains/(losses) included in earnings	(26)	28	(12)	12
Gains/(losses) included in other comprehensive income	0	0	1	(3)
Purchases	17	2	21	9
Issuances	0	0	0	0
Sales	(8)	0	(10)	0
Settlements	(3)	(2)	(13)	(15)
Transfers in to Level 3	0	2	0	2
Transfers out of Level 3	0	(2)	0	(2)
Fair value,	149	131	149	131
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	(26)	28	(12)	8
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	88	76	89	73
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	1	(3)
Purchases	17	2	21	9
Issuances	0	0	0	0
Sales	(8)	0	(10)	0
Settlements	(3)	(1)	(7)	(2)
Transfers in to Level 3	0	2	0	2
Transfers out of Level 3	0	(2)	0	(2)
Fair value,	94	77	94	77
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	2	0
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 in to Level 3	0	2	0	2
Transfers out of Level 3	0	0	0	0
Fair value,	2	2	2	2
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	12	4	12	4
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	3	0	3	0
Issuances	0	0	0	0
Sales	(3)	0	(3)	0
Settlements	0	0	0	0
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	(2)	0	(2)
Fair value,	12	2	12	2
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	45	53	48	56
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	1	(3)
Purchases	11	2	12	2
Issuances	0	0	0	0
Sales	(5)	0	(7)	0
Settlements	(1)	(1)	(4)	(1)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	50	54	50	54
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	29	19	27	13
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	0	0
Purchases	3	0	6	7
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	(2)	0	(3)	(1)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	30	19	30	19
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	81	27	73	55
Gains/(losses) included in earnings	(26)	28	(12)	12
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)	(6)	(13)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	55	54	55	54
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	(26)	28	(12)	8
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	3	11	3	12
Gains/(losses) included in earnings	(1)	2	0	2
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	(1)	(1)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	2	13	2	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	(1)	2	0	2
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	78	23	70	55
Gains/(losses) included in earnings	(25)	28	(12)	7
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)	(5)	(12)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	53	50	53	50
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	(25)	28	(12)	9
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,		(7)		(12)
Gains/(losses) included in earnings		(2)		3
Gains/(losses) 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		0
Transfers out of Level 3		0		0
Fair value,		(9)		(9)
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)		$ (3)