Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Available-for-sale securities
Amortized cost	$ 8,003	$ 7,919
Gross unrealized gains	637	560
Gross unrealized losses	16	50
Estimated fair value	8,624	8,429
Indemnity
Available-for-sale securities
Amortized cost	503	559
Gross unrealized gains	17	15
Gross unrealized losses	0	1
Estimated fair value	520	573
Indemnity \| Fixed maturities
Available-for-sale securities
Amortized cost	479	535
Gross unrealized gains	15	14
Gross unrealized losses	0	1
Estimated fair value	494	548
Indemnity \| States & political subdivisions
Available-for-sale securities
Amortized cost	205	208
Gross unrealized gains	12	13
Gross unrealized losses	0	0
Estimated fair value	217	221
Indemnity \| Corporate debt securities
Available-for-sale securities
Amortized cost	256	303
Gross unrealized gains	2	1
Gross unrealized losses	0	1
Estimated fair value	258	303
Indemnity \| Commercial mortgage-backed securities (CMBS)
Available-for-sale securities
Amortized cost	10	13
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	10	13
Indemnity \| Collateralized debt obligations (CDO)
Available-for-sale securities
Amortized cost	3	4
Gross unrealized gains	1	0
Gross unrealized losses	0	0
Estimated fair value	4	4
Indemnity \| Other debt securities
Available-for-sale securities
Amortized cost	5	7
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	5	7
Indemnity \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost	24	24
Gross unrealized gains	2	1
Gross unrealized losses	0	0
Estimated fair value	26	25
Exchange
Available-for-sale securities
Amortized cost	7,500	7,360
Gross unrealized gains	620	545
Gross unrealized losses	16	49
Estimated fair value	8,104	7,856
Exchange \| Fixed maturities
Available-for-sale securities
Amortized cost	6,939	6,829
Gross unrealized gains	555	500
Gross unrealized losses	11	37
Estimated fair value	7,483	7,292
Exchange \| U.S. government & agencies
Available-for-sale securities
Amortized cost	19	16
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Estimated fair value	20	17
Exchange \| States & political subdivisions
Available-for-sale securities
Amortized cost	1,272	1,289
Gross unrealized gains	89	91
Gross unrealized losses	0	1
Estimated fair value	1,361	1,379
Exchange \| Foreign government securities
Available-for-sale securities
Amortized cost	15	15
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value	15	15
Exchange \| Corporate debt securities
Available-for-sale securities
Amortized cost	5,277	5,144
Gross unrealized gains	443	386
Gross unrealized losses	9	31
Estimated fair value	5,711	5,499
Exchange \| Residential mortgage-backed securities (RMBS)
Available-for-sale securities
Amortized cost	169	178
Gross unrealized gains	10	11
Gross unrealized losses	0	0
Estimated fair value	179	189
Exchange \| Commercial mortgage-backed securities (CMBS)
Available-for-sale securities
Amortized cost	67	62
Gross unrealized gains	4	4
Gross unrealized losses	0	0
Estimated fair value	71	66
Exchange \| Collateralized debt obligations (CDO)
Available-for-sale securities
Amortized cost	62	66
Gross unrealized gains	5	4
Gross unrealized losses	2	5
Estimated fair value	65	65
Exchange \| Other debt securities
Available-for-sale securities
Amortized cost	58	59
Gross unrealized gains	3	3
Gross unrealized losses	0	0
Estimated fair value	61	62
Exchange \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost	561	531
Gross unrealized gains	65	45
Gross unrealized losses	5	12
Estimated fair value	$ 621	$ 564

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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