Fair Value Measurements (Using Significant Unobservable Inputs) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	$ 149	$ 131	$ 162	$ 128
Gains/(losses) included in earnings	(17)	20	(29)	32
Gains/(losses) included in other comprehensive income	0	0	1	(3)
Purchases	1	8	22	17
Issuances	0	0	0	0
Sales	(5)	0	(15)	0
Settlements	(6)	(21)	(19)	(36)
Transfers in to Level 3	2	7	2	9
Transfers out of Level 3	0	0	0	(2)
Fair value,	124	145	124	145
The amount of total (losses)/gains included in earnings attributable to assets held at the reporting date	(17)	8	(29)	15
Available-for-sale securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	94	77	89	73
Gains/(losses) included in earnings	0	0	0	0
Gains/(losses) included in other comprehensive income	0	0	1	(3)
Purchases	1	8	22	17
Issuances	0	0	0	0
Sales	(5)	0	(15)	0
Settlements	(2)	(3)	(9)	(5)
Transfers in to Level 3	2	7	2	9
Transfers out of Level 3	0	0	0	(2)
Fair value,	90	89	90	89
U.S. government and agency obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	2	2	2	0
Gains/(losses) included in earnings		0	0	0
Gains/(losses) included in other comprehensive income		0	0	0
Purchases		0	0	0
Issuances		0	0	0
Sales		0	0	0
Settlements		0	0	0
Transfers in to Level 3		0	0	2
Transfers out of Level 3		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	2	12	4
Gains/(losses) included in earnings		0	0	0
Gains/(losses) included in other comprehensive income		0	0	0
Purchases		0	3	0
Issuances		0	0	0
Sales		0	(3)	0
Settlements		0	0	0
Transfers in to Level 3	2	7	2	7
Transfers out of Level 3		0	0	(2)
Fair value,	14	9	14	9
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	50	54	48	56
Gains/(losses) included in earnings		0	0	0
Gains/(losses) included in other comprehensive income		0	1	(3)
Purchases		3	12	5
Issuances		0	0	0
Sales		0	(7)	0
Settlements	(1)	(2)	(5)	(3)
Transfers in to Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fair value,	49	55	49	55
Asset-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	30	19	27	13
Gains/(losses) included in earnings		0	0	0
Gains/(losses) included in other comprehensive income		0	0	0
Purchases	1	5	7	12
Issuances		0	0	0
Sales	(5)	0	(5)	0
Settlements	(1)	(1)	(4)	(2)
Transfers in to Level 3		0	0	0
Transfers out of Level 3		0	0	0
Fair value,	25	23	25	23
Derivative [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	55	54	73	55
Gains/(losses) included in earnings	(17)	20	(29)	32
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	(4)	(18)	(10)	(31)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	34	56	34	56
The amount of total (losses)/gains included in earnings attributable to assets held at the reporting date	(17)	8	(29)	15
Interest rate swap [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	2	13	3	12
Gains/(losses) included in earnings	(1)	3	(1)	5
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	(13)	(1)	(14)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	1	3	1	3
The amount of total (losses)/gains included in earnings attributable to assets held at the reporting date	(1)	0	(1)	(2)
Foreign currency swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,	53	50	70	55
Gains/(losses) included in earnings	(16)	8	(28)	15
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	(4)	(5)	(9)	(17)
Transfers in to Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Fair value,	33	53	33	53
The amount of total (losses)/gains included in earnings attributable to assets held at the reporting date	(16)	8	(28)	17
Embedded derivative liabilities, net
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value,		(9)		(12)
Gains/(losses) included in earnings		9		12
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,		0		0
The amount of total (losses)/gains included in earnings attributable to assets held at the reporting date		$ 0		$ 0

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Total gains or losses recognized in earnings, arising from assets and liabilities measured at fair value on a recurring basis using unobservable inputs (level 3).

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- Definition

Issues that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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Purchases that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

No definition available.

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Sales that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

+ References

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Settlements that have taken place during the period in relation to assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy.

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- Definition

Transfers into assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

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Transfers out of assets and liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

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- Definition

This element represents net asset and liability measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ References

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1a)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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