Unaudited quarterly financial data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	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
Unaudited quarterly financial data [Abstract]
Net premiums written	$ 263,773		$ 255,745		$ 270,399		$ 274,463		$ 271,409	[1]	$ 278,982		$ 295,346		$ 256,058		$ 1,064,380		$ 1,101,795		$ 1,243,027
Net premiums earned	275,741		275,094		284,454		288,546		291,125	[1]	296,496		309,174		271,952		1,123,835		1,168,747		1,302,341
Investment income, net of expenses	40,339		48,898		55,490		56,543		57,061		58,465		62,868		68,859		201,270		247,253		304,678
Loss incurred, net	482,070		462,654		459,552		310,431		448,375	[1]	384,578		320,077		454,511		1,714,707		1,607,541		3,379,444
Change in premium deficiency reserve	(11,709)		(12,388)		(11,035)		(9,018)		(18,275)	[1]	(8,887)		(10,619)		(13,566)		(44,150)		(51,347)
Underwriting and other operating expenses	50,680		52,477		54,043		57,550		53,541	[1]	57,606		54,050		59,945		214,750		225,142
Interest expense	25,142		25,761		26,326		26,042		25,770	[1]	26,702		25,099		21,018		103,271		98,589		89,266
Net income (loss)	$ (135,294)		$ (165,205)		$ (151,732)		$ (33,661)		$ (186,667)	[1]	$ (51,528)		$ 24,551		$ (150,091)		$ (485,892)		$ (363,735)		$ (1,322,277)
Income (loss) per share [Abstract]
Basic	$ (0.67)	[2]	$ (0.82)	[2]	$ (0.75)	[2]	$ (0.17)	[2]	$ (0.93)	[1],[2]	$ (0.26)	[2]	$ 0.14	[2]	$ (1.20)	[2]	$ (2.42)	[2]	$ (2.06)	[2]	$ (10.65)
Diluted	$ (0.67)	[2]	$ (0.82)	[2]	$ (0.75)	[2]	$ (0.17)	[2]	$ (0.93)	[1],[2]	$ (0.26)	[2]	$ 0.13	[2]	$ (1.20)	[2]	$ (2.42)	[2]	$ (2.06)	[2]	$ (10.65)

[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.

[2] Due to the use of weighted average shares outstanding when calculating earnings per share, the sum of the quarterly per share data may not equal the per share data for the year.