Investments (Details 5) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Realized gains (losses) on investments
Net realized investment gains (losses)	$ 418	$ (4)	$ 313
Other-than-temporary impairments on investments
Net impairment losses recognized in earnings	0	2	6
Indemnity
Limited partnership:
Net realized gains (losses)			(38)
Realized gains (losses) on investments
Net realized investment gains (losses)	5	3	(1)
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	0	1
Portion recognized in other comprehensive income	0	0	0
Net impairment losses recognized in earnings	0	0	1
Indemnity \| Fixed maturities
Available-for-sale securities:
Gross realized gains	0	2	6
Gross realized losses	0	0	(1)
Net realized gains (losses)	0	2	5
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	0	1
Indemnity \| Equity securities
Available-for-sale securities:
Gross realized gains	0	3	1
Gross realized losses	0	0	0
Net realized gains (losses)	0	3	1
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	0	0
Indemnity \| Common stock
Trading securities:
Gross realized gains	9	2	6
Gross realized losses	(1)	(1)	(1)
Valuation adjustments	(3)	(3)	0
Net realized gains (losses)	5	(2)	5
Indemnity \| Limited partnership
Limited partnership:
Gross realized gains	0	0	0
Gross realized losses	0	0	(12)
Net realized gains (losses)	0	0	(12)
Realized gains (losses) on investments
Number of limited partnerships sold			9
Exchange
Limited partnership:
Net realized gains (losses)			(175)
Realized gains (losses) on investments
Net realized investment gains (losses)	413	(7)	314
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	2	5
Portion recognized in other comprehensive income	0	0	0
Net impairment losses recognized in earnings	0	2	5
Exchange \| Fixed maturities
Available-for-sale securities:
Gross realized gains	78	77	49
Gross realized losses	(20)	(29)	(24)
Net realized gains (losses)	58	48	25
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	0	4
Exchange \| Equity securities
Available-for-sale securities:
Gross realized gains	17	28	14
Gross realized losses	(8)	(1)	(3)
Net realized gains (losses)	9	27	11
Other-than-temporary impairments on investments
Total other-than-temporary impairments	0	2	1
Exchange \| Common stock
Trading securities:
Gross realized gains	213	271	205
Gross realized losses	(88)	(106)	(135)
Valuation adjustments	221	(247)	254
Net realized gains (losses)	346	(82)	324
Exchange \| Limited partnership
Limited partnership:
Gross realized gains	0	0	0
Gross realized losses	0	0	(46)
Net realized gains (losses)	$ 0	$ 0	$ (46)
Realized gains (losses) on investments
Number of limited partnerships sold			11

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