Investments - Amortized Cost and Fair Values of Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	$ 39,377	$ 39,217
Gross Unrealized Gains	4,499	4,719
Gross Unrealized Losses	273	273
Estimated Fair Value	43,603	43,663
Unrealized OTTI Losses (Gains)	(52)	(31)
Fixed maturity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	38,668	38,324
Gross Unrealized Gains	4,424	4,618
Gross Unrealized Losses	180	177
Estimated Fair Value	42,912	42,765
Unrealized OTTI Losses (Gains)	(52)	(31)
Corporate and other bonds [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	19,747	19,530
Gross Unrealized Gains	2,562	2,698
Gross Unrealized Losses	18	21
Estimated Fair Value	22,291	22,207
Unrealized OTTI Losses (Gains)
States, municipalities and political subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	9,599	9,372
Gross Unrealized Gains	1,408	1,455
Gross Unrealized Losses	59	44
Estimated Fair Value	10,948	10,783
Unrealized OTTI Losses (Gains)
Residential mortgage-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	5,518	5,745
Gross Unrealized Gains	235	246
Gross Unrealized Losses	70	71
Estimated Fair Value	5,683	5,920
Unrealized OTTI Losses (Gains)	(49)	(28)
Commercial mortgage-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	1,853	1,692
Gross Unrealized Gains	147	147
Gross Unrealized Losses	10	17
Estimated Fair Value	1,990	1,822
Unrealized OTTI Losses (Gains)	(3)	(3)
Other asset-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	932	929
Gross Unrealized Gains	23	23
Gross Unrealized Losses
Estimated Fair Value	955	952
Unrealized OTTI Losses (Gains)
Total asset-backed [Member] \| Fixed maturity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	8,303	8,366
Gross Unrealized Gains	405	416
Gross Unrealized Losses	80	88
Estimated Fair Value	8,628	8,694
Unrealized OTTI Losses (Gains)	(52)	(31)
U.S. Treasury and obligations of government - sponsored enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	170	172
Gross Unrealized Gains	11	11
Gross Unrealized Losses		1
Estimated Fair Value	181	182
Unrealized OTTI Losses (Gains)
Foreign government [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	528	588
Gross Unrealized Gains	24	25
Gross Unrealized Losses
Estimated Fair Value	552	613
Unrealized OTTI Losses (Gains)
Redeemable preferred stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	123	113
Gross Unrealized Gains	14	13
Gross Unrealized Losses	1	1
Estimated Fair Value	136	125
Unrealized OTTI Losses (Gains)
Total fixed maturities available-for-sale [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	38,470	38,141
Gross Unrealized Gains	4,424	4,618
Gross Unrealized Losses	158	155
Estimated Fair Value	42,736	42,604
Unrealized OTTI Losses (Gains)	(52)	(31)
Fixed maturities, trading [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	198	183
Gross Unrealized Gains
Gross Unrealized Losses	22	22
Estimated Fair Value	176	161
Unrealized OTTI Losses (Gains)
Common Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	38	38
Gross Unrealized Gains	17	14
Gross Unrealized Losses
Estimated Fair Value	55	52
Unrealized OTTI Losses (Gains)
Preferred Stock [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	139	190
Gross Unrealized Gains	7	7
Gross Unrealized Losses
Estimated Fair Value	146	197
Unrealized OTTI Losses (Gains)
Total equity securities available-for-sale [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	177	228
Gross Unrealized Gains	24	21
Gross Unrealized Losses
Estimated Fair Value	201	249
Unrealized OTTI Losses (Gains)
Equity securities, trading [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	532	665
Gross Unrealized Gains	51	80
Gross Unrealized Losses	93	96
Estimated Fair Value	490	649
Unrealized OTTI Losses (Gains)
Equity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or Amortized Cost	709	893
Gross Unrealized Gains	75	101
Gross Unrealized Losses	93	96
Estimated Fair Value	691	898
Unrealized OTTI Losses (Gains)

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.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- 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.

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