Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 128	$ 79
Gains/(losses) included in earnings	(18)	(9)
Gains/(losses) included in other comprehensive income	(3)	0
Purchases	7	1
Issuances	0	0
Sales	0	(1)
Settlements	(11)	(6)
Transfers into Level 3	0	27
Transfers out of Level 3	0	0
Fair value,	103	91
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	(21)	(9)
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	73	21
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	(3)	0
Purchases	7	1
Issuances	0	0
Sales	0	(1)
Settlements	(1)	0
Transfers into Level 3	0	27
Transfers out of Level 3	0	0
Fair value,	76	48
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,	5	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,	5	7
Non-agency commercial mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	51	15
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	(3)	0
Purchases	0	1
Issuances	0	0
Sales	0	(1)
Settlements	0	0
Transfers into Level 3	0	10
Transfers out of Level 3	0	0
Fair value,	48	25
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	13	1
Gains/(losses) included in earnings	0	0
Gains/(losses) included in other comprehensive income	0	0
Purchases	7	0
Issuances	0	0
Sales	0	0
Settlements	(1)	0
Transfers into Level 3	0	11
Transfers out of Level 3	0	0
Fair value,	19	12
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	55	58
Gains/(losses) included in earnings	(18)	(9)
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(10)	(6)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value,	27	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	(21)	(9)
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	12	13
Gains/(losses) included in earnings	(1)	(1)
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	0	(1)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value,	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	(1)	(1)
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	55	69
Gains/(losses) included in earnings	(22)	(5)
Gains/(losses) included in other comprehensive income	0	0
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	(10)	(5)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Fair value,	23	59
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	(19)	(5)
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	(12)	(24)
Gains/(losses) included in earnings	5	(3)
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	0
Fair value,	(7)	(27)
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)	$ (3)