Reinsurance (Details) (USD $)	3 Months Ended									12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010		Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Reinsurance [Abstract]
First aggregate layer of loss under aggregate excess of loss agreements, minimum (in hundredths)										4.00%
First aggregate layer of loss under aggregate excess of loss agreements, maximum (in hundredths)										5.00%
Second aggregate layer of loss under aggregate excess of loss agreements, captives, minimum (in hundredths)										5.00%
Second aggregate layer of loss under aggregate excess of loss agreements, captives, maximum (in hundredths)										10.00%
Premium cessions as a percentage of the direct premium, minimum (in hundredths)										25.00%
Premium cessions as a percentage of the direct premium, maximum (in hundredths)										40.00%
Captive's portion of both premiums and losses under quota share arrangement, minimum (in hundredths)										25.00%
Captive's portion of both premiums and losses under quota share arrangement, maximum (in hundredths)										50.00%
New business cede rate, maximum (in hundredths)										25.00%
Reinsurance recoverable on loss reserves	$ 154,607,000				$ 275,290,000					$ 154,607,000	$ 275,290,000	$ 332,227,000	$ 232,988,000
Reinsurance recoverable on loss reserves related to captive agreements	142,000,000				248,000,000					142,000,000	248,000,000
Fair value of trust fund assets under our captive agreements	359,000,000				484,000,000					359,000,000	484,000,000
Fair value of trust fund assets under captive agreements, no reinsurance recoverable on loss reserves	27,000,000				26,000,000					27,000,000	26,000,000
Trust fund assets transferred to us as a result of captive terminations										39,000,000	38,000,000
Number of aggregate excess of loss agreements with special purpose reinsurance companies	3									3
Risk in force amount ceded to date under aggregate excess of loss reinsurance agreements	130,000,000									130,000,000
Number of special purpose reinsurance companies to which risk in force has been ceded	3									3
Number of aggregate excess of loss reinsurance agreements terminated with special purpose reinsurance companies													1
Risk in force amount ceded under remaining aggregate excess of loss reinsurance agreements	23,800,000									23,800,000
Number of domestic reinsurers	1									1
Premiums earned [Abstract]
Direct										1,170,868,000	1,236,949,000	1,406,977,000
Assumed										3,891,000	3,091,000	3,339,000
Ceded										(50,924,000)	(71,293,000)	(107,975,000)
Net premiums earned	275,741,000	275,094,000	284,454,000	288,546,000	291,125,000	[1]	296,496,000	309,174,000	271,952,000	1,123,835,000	1,168,747,000	1,302,341,000
Losses incurred [Abstract]
Direct										1,775,122,000	1,716,538,000	3,637,706,000
Assumed										5,229,000	4,128,000	4,290,000
Ceded										(65,644,000)	(113,125,000)	(262,552,000)
Net losses incurred	482,070,000	462,654,000	459,552,000	310,431,000	448,375,000	[1]	384,578,000	320,077,000	454,511,000	1,714,707,000	1,607,541,000	3,379,444,000
Unearned premium returned under assumed reinsurance agreement terminated by our Australian writing company		7,000,000
Qualifying new insurance for which reinsurance agreement was effective, maximum													50,000,000,000
Number of optional extensions on reinsurance agreement													2
Term of optional extensions under reinsurance agreement (in years)													1Y
Reinsurance fee										$ 0	$ 0	$ 26,407,000

[1] In prior periods, the liability associated with premium to be returned on claim payments is included in loss reserves and changes to this estimate affect losses incurred. See Note 3 - Summary of significant accounting policies - Revenue recognition.