Investments (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Limited partnership:
Net realized gains (losses)	$ 8
Other-than-temporary impairments on investments
Net impairment losses recognized in earnings	(2)	(6)	(126)
Realized gains (losses) on investments
Net realized investment gains (losses)	(4)	313	412
Indemnity
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	(1)	(12)
Portion recognized in other comprehensive income	0	0	0
Realized gains (losses) on investments
Net realized investment gains (losses)	3	(1)	10
Indemnity \| Fixed maturities
Available-for-sale securities:
Gross realized gains	2	6	5
Gross realized losses	0	(1)	(4)
Net realized gains	2	5	1
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	(1)	(7)
Indemnity \| Equity securities
Available-for-sale securities:
Gross realized gains	3	1	8
Gross realized losses	0	0	(7)
Net realized gains	3	1	1
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	0	(5)
Indemnity \| Common stock
Trading securities:
Gross realized gains	2	6	2
Gross realized losses	(1)	(1)	(5)
Valuation adjustments	(3)	0	11
Net realized gains (losses)	(2)	5	8
Indemnity \| Limited partnerships
Limited partnership:
Gross realized gains	0	0	0
Gross realized losses	0	(12)	0
Net realized gains (losses)	0	(12)	0
Realized gains (losses) on investments
Number of limited partnerships sold (in counts)		9
Exchange
Other-than-temporary impairments on investments
Total other-than-temporary impairments	(2)	(5)	(114)
Portion recognized in other comprehensive income	0	0	0
Realized gains (losses) on investments
Net realized investment gains (losses)	(7)	314	402
Exchange \| Fixed maturities
Available-for-sale securities:
Gross realized gains	77	49	22
Gross realized losses	(29)	(24)	(37)
Net realized gains	48	25	(15)
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	(4)	(54)
Exchange \| Equity securities
Available-for-sale securities:
Gross realized gains	28	14	39
Gross realized losses	(1)	(3)	(26)
Net realized gains	27	11	13
Other-than-temporary impairments on investments
Total other-than-temporary impairments	(2)	(1)	(60)
Exchange \| Common stock
Trading securities:
Gross realized gains	271	205	143
Gross realized losses	(106)	(135)	(203)
Valuation adjustments	(247)	254	464
Net realized gains (losses)	(82)	324	404
Exchange \| Limited partnerships
Limited partnership:
Gross realized gains	0	0	0
Gross realized losses	0	(46)	0
Net realized gains (losses)	$ 0	$ (46)	$ 0
Realized gains (losses) on investments
Number of limited partnerships sold (in counts)		11

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