Quarterly Results of Operations (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of unaudited quarterly results of operations
Net premiums earned	$ 193,694	$ 185,275	$ 176,732	$ 177,579	$ 164,874	$ 182,085	$ 175,293	$ 171,899	$ 733,281	$ 694,149	$ 699,731
Net losses and loss adjustment expenses:
Current year	156,585	147,093	143,668	139,660	149,157	145,027	139,057	138,654	587,007	571,891	545,168
Prior year	(49,292)	(29,011)	(36,769)	(28,705)	(56,330)	(36,221)	(35,115)	(33,514)	(143,778)	(161,180)	(182,084)
Net income	$ 54,848	$ 33,834	$ 43,081	$ 19,317	$ 34,948	$ 10,276	$ 33,158	$ 37,814	$ 151,081	$ 116,197	$ 196,565
Basic earnings per share (USD per share)	$ 1.03	$ 0.64	$ 0.81	$ 0.36	$ 0.66	$ 0.19	$ 0.60	$ 0.67	$ 2.84	$ 2.12	$ 3.32
Diluted earnings per share (USD per share)	$ 1.02	$ 0.63	$ 0.81	$ 0.36	$ 0.65	$ 0.19	$ 0.60	$ 0.67	$ 2.83	$ 2.11	$ 3.30