Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Unaudited Quarterly Financial Data [Abstract]
Net premiums earned	$ 213,589		$ 209,035		$ 207,486		$ 214,261		$ 226,358		$ 231,857		$ 237,777		$ 247,059		$ 844,371	$ 943,051	$ 1,033,170
Investment income, net of expenses	23,956		22,355		21,180		20,156		21,278		20,250		20,883		18,328		87,647	80,739	121,640
Realized (losses) gains	434		632		522		(231)		2,126		(139)		2,485		1,259		1,357	5,731	195,409
Other revenue	2,385		3,093		2,048		896		2,179		2,481		2,715		2,539		8,422	9,914	28,145
Loss incurred, net	117,074		115,254		141,141		122,608		196,055		180,189		196,274		266,208		496,077	838,726	2,067,253
Underwriting and other expenses, net	48,181		47,595		43,455		51,766		56,062		61,810		54,221		74,768		190,997	246,861
Provision for (benefit from) income taxes (note 14)	681		249		1,118		726		1,231		336		990		1,139		2,774	3,696	(1,565)
Net income (loss)	$ 74,428		$ 72,017		$ 45,522		$ 59,982		$ (1,407)		$ 12,114		$ 12,375		$ (72,930)		$ 251,949	$ (49,848)	$ (927,079)
Income (loss) per share [Abstract]
Basic (in dollars per share)	$ 0.22	[1],[2]	$ 0.21	[1],[2]	$ 0.13	[1],[2]	$ 0.18	[1],[2]	$ 0	[1]	$ 0.04	[1]	$ 0.04	[1]	$ (0.31)	[1]	$ 0.74	$ (0.16)	$ (4.59)
Diluted (in dollars per share)	$ 0.19	[1],[2]	$ 0.18	[1],[2]	$ 0.12	[1],[2]	$ 0.15	[1],[2]	$ 0	[1]	$ 0.04	[1]	$ 0.04	[1]	$ (0.31)	[1]	$ 0.64	$ (0.16)	$ (4.59)

[1] 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.

[2] In periods where convertible debt instruments are dilutive to earnings per share the "if-converted" method of computing diluted EPS requires an interest expense adjustment, net of tax, to net income available to shareholders. This adjustment has not been reflected in the Unaudited Quarterly Financial Data presented. See Note 3 - "Summary of Significant Accounting Policies" for further discussion.