Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available for Sale Securities
Amortized cost	$ 70,957	$ 71,915
Gross unrealized gains	5,231	5,632
Gross unrealized losses	(382)	(530)
Fair value	75,806	77,017
U.S. government and agencies
Schedule of Available for Sale Securities
Amortized cost	3,960	4,387
Gross unrealized gains	297	326
Fair value	4,257	4,713
Municipal
Schedule of Available for Sale Securities
Amortized cost	10,933	12,139
Gross unrealized gains	975	1,038
Gross unrealized losses	(46)	(108)
Fair value	11,862	13,069
Corporate
Schedule of Available for Sale Securities
Amortized cost	46,267	44,943
Gross unrealized gains	3,428	3,721
Gross unrealized losses	(128)	(127)
Fair value	49,567	48,537
Foreign government
Schedule of Available for Sale Securities
Amortized cost	2,165	2,290
Gross unrealized gains	203	228
Gross unrealized losses	(3)	(1)
Fair value	2,365	2,517
Asset-backed securities ("ABS")
Schedule of Available for Sale Securities
Amortized cost	3,579	3,623
Gross unrealized gains	104	108
Gross unrealized losses	(86)	(107)
Fair value	3,597	3,624
Residential mortgage-backed securities ("RMBS")
Schedule of Available for Sale Securities
Amortized cost	2,685	3,000
Gross unrealized gains	142	142
Gross unrealized losses	(77)	(110)
Fair value	2,750	3,032
Commercial mortgage-backed securities ("CMBS")
Schedule of Available for Sale Securities
Amortized cost	1,345	1,510
Gross unrealized gains	78	65
Gross unrealized losses	(42)	(77)
Fair value	1,381	1,498
Redeemable preferred stock
Schedule of Available for Sale Securities
Amortized cost	23	23
Gross unrealized gains	4	4
Fair value	$ 27	$ 27

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

-Paragraph 15C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

-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 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

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (b)-(g)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-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.

+ 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