Analysis of Cost or Amortized Cost, Gross Unrealized Gains and Losses and Fair Value for Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	$ 10,548	$ 10,246
Gross unrealized gains	1,923	1,542
Gross unrealized losses	6	53
Fair value	12,465	11,735
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	8,233	8,084
Gross unrealized gains	887	713
Gross unrealized losses	4	18
Fair value	9,116	8,779
Debt Securities \| US States and Political Subdivisions Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	3,023	3,006
Gross unrealized gains	265	246
Fair value	3,288	3,252
Debt Securities \| Convertibles and Bonds with Warrants Attached
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	31	59
Fair value	31	59
Debt Securities \| US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	7	6
Gross unrealized gains	1	1
Fair value	8	7
Debt Securities \| US Government-sponsored Enterprises Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	116	159
Gross unrealized gains		1
Fair value	116	160
Debt Securities \| Foreign Government
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	3	3
Fair value	3	3
Debt Securities \| Corporate Bond Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	5,053	4,851
Gross unrealized gains	621	465
Gross unrealized losses	4	18
Fair value	5,670	5,298
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,315	2,162
Gross unrealized gains	1,036	829
Gross unrealized losses	2	35
Fair value	3,349	2,956
Equity Securities \| Common Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	2,212	2,088
Gross unrealized gains	1,000	801
Gross unrealized losses	2	35
Fair value	3,210	2,854
Equity Securities \| Preferred Stock
Schedule of Available-for-sale Securities [Line Items]
Cost or amortized cost	103	74
Gross unrealized gains	36	28
Fair value	$ 139	$ 102

Amount of investment in debt and equity 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 Accounting Standards Codification

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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

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

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

-Publisher FASB

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Reference 8: 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 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ 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