Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Gross written premiums	$ 1,453,502	$ 1,432,353	$ 1,370,517
Revenues:
Net written premiums	1,043,860	1,000,138	887,922
Change in unearned premiums	(59,773)	(64,243)	(45,983)
Net earned premiums	984,087	935,895	841,939
Net investment income	68,718	64,168	56,251
Total other-than-temporary impairment losses	(1,870)	137	(2,393)
Portion of loss recognized in other comprehensive income (before tax)	172	(137)
Net other-than-temporary impairment losses recognized in earnings	(1,698)	0	(2,393)
Net realized gains (losses)	8,373	12,812	22,939
Other income (expense)	(491)	10,656	(1,172)
Total revenues	1,058,989	1,023,531	917,564
Expenses:
Net losses and loss adjustment expenses	572,598	545,229	518,961
Commission expenses	129,977	125,528	113,494
Other operating expenses	223,516	196,825	164,434
Call premium on Senior notes			17,895
Interest expense	15,424	15,413	10,507
Total expenses	941,515	882,995	825,291
Income before income taxes	117,474	140,536	92,273
Income tax expense (benefit)	36,417	45,207	28,807
Net income	$ 81,057	$ 95,329	$ 63,466
Net income per common share:
Basic	$ 5.63	$ 6.69	$ 4.49
Diluted	$ 5.47	$ 6.51	$ 4.42
Average common shares outstanding:
Basic	14,392,522	14,259,768	14,133,925
Diluted	14,825,745	14,646,369	14,345,553