Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Available-for-sale securities
Amortized cost	$ 8,552	$ 8,378
Gross unrealized gains	766	790
Gross unrealized losses	14	9
Estimated fair value, fixed maturities	8,312	8,159
Estimated fair value, equity securities	992	1,000
Estimated fair value	9,304	9,159
Indemnity
Available-for-sale securities
Amortized cost, fixed maturities	406	437
Amortized cost, equity securities	47	54
Amortized cost	453	491
Gross unrealized gains	17	17
Gross unrealized losses	0	1
Estimated fair value, fixed maturities	421	452
Estimated fair value, equity securities	49	55
Estimated fair value	470	507
Indemnity \| Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	406	437
Gross unrealized gains	15	15
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	421	452
Indemnity \| States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	164	172
Gross unrealized gains	12	13
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	176	185
Indemnity \| Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	240	259
Gross unrealized gains	3	2
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	243	261
Indemnity \| Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities		3
Gross unrealized gains		0
Gross unrealized losses		0
Estimated fair value, fixed maturities		3
Indemnity \| Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	2	3
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	2	3
Indemnity \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	21	28
Gross unrealized gains	2	2
Gross unrealized losses	0	1
Estimated fair value, equity securities	23	29
Indemnity \| Common stock
Available-for-sale securities
Amortized cost, equity securities	26	26
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Estimated fair value, equity securities	26	26
Exchange
Available-for-sale securities
Amortized cost, fixed maturities	7,242	7,016
Amortized cost, equity securities	857	871
Amortized cost	8,099	7,887
Gross unrealized gains	749	773
Gross unrealized losses	14	8
Estimated fair value, fixed maturities	7,891	7,707
Estimated fair value, equity securities	943	945
Estimated fair value	8,834	8,652
Exchange \| Fixed maturities
Available-for-sale securities
Amortized cost, fixed maturities	7,242	7,016
Gross unrealized gains	658	696
Gross unrealized losses	9	5
Estimated fair value, fixed maturities	7,891	7,707
Exchange \| U.S. government & agencies
Available-for-sale securities
Amortized cost, fixed maturities	185	190
Gross unrealized gains	1	2
Gross unrealized losses	1	1
Estimated fair value, fixed maturities	185	191
Exchange \| States & political subdivisions
Available-for-sale securities
Amortized cost, fixed maturities	1,204	1,218
Gross unrealized gains	96	103
Gross unrealized losses	1	0
Estimated fair value, fixed maturities	1,299	1,321
Exchange \| Foreign government securities
Available-for-sale securities
Amortized cost, fixed maturities	28	15
Gross unrealized gains	1	1
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	29	16
Exchange \| Corporate debt securities
Available-for-sale securities
Amortized cost, fixed maturities	5,442	5,211
Gross unrealized gains	538	569
Gross unrealized losses	6	3
Estimated fair value, fixed maturities	5,974	5,777
Exchange \| Residential mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	229	226
Gross unrealized gains	6	6
Gross unrealized losses	1	1
Estimated fair value, fixed maturities	234	231
Exchange \| Commercial mortgage-backed securities
Available-for-sale securities
Amortized cost, fixed maturities	52	62
Gross unrealized gains	4	5
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	56	67
Exchange \| Collateralized debt obligations
Available-for-sale securities
Amortized cost, fixed maturities	39	43
Gross unrealized gains	7	6
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	46	49
Exchange \| Other debt securities
Available-for-sale securities
Amortized cost, fixed maturities	63	51
Gross unrealized gains	5	4
Gross unrealized losses	0	0
Estimated fair value, fixed maturities	68	55
Exchange \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost, equity securities	541	555
Gross unrealized gains	91	77
Gross unrealized losses	1	1
Estimated fair value, equity securities	631	631
Exchange \| Common stock
Available-for-sale securities
Amortized cost, equity securities	316	316
Gross unrealized gains	0	0
Gross unrealized losses	4	2
Estimated fair value, equity securities	$ 312	$ 314

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

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

+ References

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+ Details

- Definition

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

+ References

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+ Details

- Definition

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.

+ 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