Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	$ 40,692	$ 41,194
Gross unrealized gains	3,852	4,108
Gross unrealized losses	(321)	(426)
Fair value	44,223	44,876
U.S. government and agencies
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,776	2,137
Gross unrealized gains	221	242
Fair value	1,997	2,379
Municipal
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	3,983	4,153
Gross unrealized gains	615	612
Gross unrealized losses	(29)	(61)
Fair value	4,569	4,704
Corporate
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	29,314	28,748
Gross unrealized gains	2,661	2,896
Gross unrealized losses	(115)	(113)
Fair value	31,860	31,531
Foreign government
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,007	1,017
Gross unrealized gains	150	164
Gross unrealized losses	(2)	(1)
Fair value	1,155	1,180
Asset-backed securities ("ABS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,748	1,921
Gross unrealized gains	47	49
Gross unrealized losses	(85)	(105)
Fair value	1,710	1,865
Residential mortgage-backed securities ("RMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,589	1,778
Gross unrealized gains	81	82
Gross unrealized losses	(48)	(69)
Fair value	1,622	1,791
Commercial mortgage-backed securities ("CMBS")
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	1,260	1,425
Gross unrealized gains	74	60
Gross unrealized losses	(42)	(77)
Fair value	1,292	1,408
Redeemable preferred stock
Amortized cost, gross unrealized gains and losses and fair value for fixed income securities
Amortized cost	15	15
Gross unrealized gains	3	3
Fair value	$ 18	$ 18

Amount of debt securities categorized neither as held-to-maturity nor trading.

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-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.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details