Premium deficiency reserves (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Premium Deficiency Reserve [Abstract]
Premium deficiency reserve, discount rate (in hundredths)	2.30%		2.50%
Present value of expected future premium	$ 494,000,000		$ 506,000,000		$ 427,000,000
Present value of expected future paid losses and expenses	(1,455,000,000)		(1,760,000,000)		(2,157,000,000)
Net present value of future cash flows	(961,000,000)		(1,254,000,000)		(1,730,000,000)
Established loss reserves	826,000,000		1,075,000,000		1,537,000,000
Net deficiency	(134,817,000)		(178,967,000)		(193,186,000)
Change in premium deficiency reserve, excluding prior period adjustments	(44,000,000)		(14,000,000)		(261,000,000)
Premium Deficiency Reserve [Roll Forward]
Premium Deficiency Reserve at beginning of period	(178,967,000)		(193,186,000)		(454,000,000)
Adjustment to premium deficiency reserve	0	[1]	(37,000,000)	[1]	0	[1]
Adjusted premium deficiency reserve at beginning of period	(179,000,000)		(230,000,000)		(454,000,000)
Paid claims and loss adjustment expenses	334,000,000		426,000,000		584,000,000
Decrease in loss reserves	(249,000,000)		(425,000,000)		(360,000,000)
Premium earned	(120,000,000)		(128,000,000)		(156,000,000)
Effects of present valuing on future premiums, losses and expenses	(8,000,000)		(25,000,000)		21,000,000
Change in premium deficiency reserve to reflect actual premium, losses and expenses recognized	(43,000,000)		(152,000,000)		89,000,000
Change in premium deficiency reserve to reflect change in assumptions relating to future premiums, losses expenses and discount rate	87,000,000	[2]	203,000,000	[2]	172,000,000	[2]
Premium Deficiency Reserve at end of period	$ (134,817,000)		$ (178,967,000)		$ (193,186,000)

[1] In periods prior to 2010 an estimate of premium to be refunded in conjunction with claim payments was included in Loss Reserves. In 2010, we separately stated this liability in Premium deficency reserve on the consolidated balance sheet. (See Note 3 - "Summary of significant accounting policies - Revenue recognition")

[2] A positive number for changes in assumptions relating to premiums, losses, expenses and discount rate indicates a redundancy of prior premium deficiency reserves.