Investments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Available-for-sale securities
Amortized cost	$ 7,919	$ 7,643
Gross unrealized gains	560	533
Gross unrealized losses	50	39
Estimated fair value	8,429	8,137
Securities lending program
Collateral as percentage of initial value of securities loaned	102.00%
Collateral as percentage of ongoing value of securities loaned	100.00%
Indemnity
Available-for-sale securities
Amortized cost	559	277
Gross unrealized gains	15	13
Gross unrealized losses	1	2
Estimated fair value	573	288
Indemnity \| Fixed maturities
Available-for-sale securities
Amortized cost	535	257
Gross unrealized gains	14	9
Gross unrealized losses	1	2
Estimated fair value	548	264
Indemnity \| U.S. government & agencies
Available-for-sale securities
Amortized cost		25
Gross unrealized gains		0
Gross unrealized losses		0
Estimated fair value		25
Indemnity \| States & political subdivisions
Available-for-sale securities
Amortized cost	208	193
Gross unrealized gains	13	6
Gross unrealized losses	0	2
Estimated fair value	221	197
Indemnity \| Corporate debt securities
Available-for-sale securities
Amortized cost	303	36
Gross unrealized gains	1	2
Gross unrealized losses	1	0
Estimated fair value	303	38
Indemnity \| Commercial mortgage-backed securities (CMBS)
Available-for-sale securities
Amortized cost	13
Gross unrealized gains	0
Gross unrealized losses	0
Estimated fair value	13
Indemnity \| Collateralized debt obligations (CDO)
Available-for-sale securities
Amortized cost	4	3
Gross unrealized gains	0	1
Gross unrealized losses	0	0
Estimated fair value	4	4
Indemnity \| Other debt securities
Available-for-sale securities
Amortized cost	7
Gross unrealized gains	0
Gross unrealized losses	0
Estimated fair value	7
Indemnity \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost	24	20
Gross unrealized gains	1	4
Gross unrealized losses	0	0
Estimated fair value	25	24
Exchange
Available-for-sale securities
Amortized cost	7,360	7,366
Gross unrealized gains	545	520
Gross unrealized losses	49	37
Estimated fair value	7,856	7,849
Exchange \| Fixed maturities
Available-for-sale securities
Amortized cost	6,829	6,863
Gross unrealized gains	500	446
Gross unrealized losses	37	30
Estimated fair value	7,292	7,279
Exchange \| U.S. government & agencies
Available-for-sale securities
Amortized cost	16	85
Gross unrealized gains	1	2
Gross unrealized losses	0	0
Estimated fair value	17	87
Exchange \| States & political subdivisions
Available-for-sale securities
Amortized cost	1,289	1,437
Gross unrealized gains	91	43
Gross unrealized losses	1	9
Estimated fair value	1,379	1,471
Exchange \| Foreign government securities
Available-for-sale securities
Amortized cost	15	20
Gross unrealized gains	0	1
Gross unrealized losses	0	0
Estimated fair value	15	21
Exchange \| Corporate debt securities
Available-for-sale securities
Amortized cost	5,144	4,900
Gross unrealized gains	386	377
Gross unrealized losses	31	14
Estimated fair value	5,499	5,263
Exchange \| Residential mortgage-backed securities (RMBS)
Available-for-sale securities
Amortized cost	178	216
Gross unrealized gains	11	9
Gross unrealized losses	0	1
Estimated fair value	189	224
Exchange \| Commercial mortgage-backed securities (CMBS)
Available-for-sale securities
Amortized cost	62	82
Gross unrealized gains	4	5
Gross unrealized losses	0	1
Estimated fair value	66	86
Exchange \| Collateralized debt obligations (CDO)
Available-for-sale securities
Amortized cost	66	69
Gross unrealized gains	4	6
Gross unrealized losses	5	5
Estimated fair value	65	70
Exchange \| Other debt securities
Available-for-sale securities
Amortized cost	59	54
Gross unrealized gains	3	3
Gross unrealized losses	0	0
Estimated fair value	62	57
Exchange \| Nonredeemable preferred stock
Available-for-sale securities
Amortized cost	531	503
Gross unrealized gains	45	74
Gross unrealized losses	12	7
Estimated fair value	$ 564	$ 570

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

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