Fair Value Measurements (Changes in Level 3 Instruments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3	$ 2,907	$ 2,990
Domestic Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		3,251	3,552	3,235
Net realized/unrealized gains (losses) included in earnings	(258)	26
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	218	37
Purchases	61	13
Sales	(6)	(31)
Settlements	(45)	(16)
Transfers into Level 3	93	0
Transfers out of Level 3	(73)	(13)
Net change in unrealized gains (losses) relating to instruments still held	0	0
Residential Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		107	100	41
Net realized/unrealized gains (losses) included in earnings	0	(3)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(3)	3
Purchases	0	0
Sales	0	0
Settlements	(1)	(1)
Transfers into Level 3	0	68
Transfers out of Level 3	0	(1)
Net change in unrealized gains (losses) relating to instruments still held	0	0
Commercial Mortgage Backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		1	6	4
Net realized/unrealized gains (losses) included in earnings	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0
Purchases	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	(3)
Net change in unrealized gains (losses) relating to instruments still held	0	0
US States and Political Subdivisions Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		79	77	77
Net realized/unrealized gains (losses) included in earnings	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	2
Purchases	4	0
Sales	0	0
Settlements	(1)	0
Transfers into Level 3	10	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
Foreign Corporate Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		1,249	1,212	1,204
Net realized/unrealized gains (losses) included in earnings	8	(9)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	13	60
Purchases	126	10
Sales	(3)	0
Settlements	(35)	(26)
Transfers into Level 3	15	14
Transfers out of Level 3	0	(4)
Net change in unrealized gains (losses) relating to instruments still held	0	0
Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		4,404	5,023	4,040
Net realized/unrealized gains (losses) included in earnings	1	(4)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(2)	42
Purchases	144	341
Sales	0	(31)
Settlements	(262)	0
Transfers into Level 3	12	16
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
Foreign Government Debt Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		52	42	84
Net realized/unrealized gains (losses) included in earnings	0	(34)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(1)	35
Purchases	0	52
Sales	0	(71)
Settlements	0	(14)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
US Treasury and Government [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		260	277	253
Net realized/unrealized gains (losses) included in earnings	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(13)	7
Purchases	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
Retained Interest [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		34	83	35
Net realized/unrealized gains (losses) included in earnings	3	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	10	(4)
Purchases	0	5
Sales	0	(1)
Settlements	(5)	(1)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
Available-for-sale Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		15	13	17
Net realized/unrealized gains (losses) included in earnings	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	(1)
Purchases	0	0
Sales	0	0
Settlements	0	(1)
Transfers into Level 3	0	0
Transfers out of Level 3	(2)	0
Net change in unrealized gains (losses) relating to instruments still held	0	0
Derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		117	262	141
Net realized/unrealized gains (losses) included in earnings	(38)	(25)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	1
Purchases	(1)	0
Sales	0	0
Settlements	(53)	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	(7)	(26)
Other 1 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		390		388
Net realized/unrealized gains (losses) included in earnings	(2)	2
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	432
Purchases	33	0
Sales	(54)	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Net change in unrealized gains (losses) relating to instruments still held	(1)	2
Total [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3		9,959	11,079	9,519
Net realized/unrealized gains (losses) included in earnings	(286)	(47)
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	222	182
Purchases	367	421
Sales	(63)	(134)
Settlements	(402)	(59)
Transfers into Level 3	130	98
Transfers out of Level 3	(75)	(21)
Net change in unrealized gains (losses) relating to instruments still held	$ (8)	$ (24)

This item represents the amount of the total change in unrealized (holding) gains or losses for the period which are included in the statement of income (or changes in net assets) in investment income. Such unrealized (holding) gains or losses relate to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

Amount of gain (loss) recognized in earnings, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

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Amount of gain (loss) recognized in other comprehensive income, arising from assets measured at fair value on a recurring basis using unobservable inputs (level 3).

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- Definition

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

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- Definition

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

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

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- Definition

Transfers into assets 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 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 an asset measured at fair value using significant unobservable inputs (Level 3) which is required for reconciliation purposes of beginning and ending balances.

+ 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)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 157

-Paragraph 32

-Subparagraph c

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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