Consolidated Statements of Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Gross written premiums	$ 413,877	$ 396,460
Revenues:
Net written premiums	319,820	288,958
Change in unearned premiums	(55,462)	(52,826)
Net earned premiums	264,358	236,132
Net investment income	19,594	16,253
Total other-than-temporary impairment losses	(109)	23
Portion of loss recognized in other comprehensive income (before tax)	109	(23)
Net other-than-temporary impairment losses recognized in earnings	0	0
Net realized gains (losses)	1,597	5,596
Other income (expenses)	2,549	2,242
Total revenues	288,098	260,223
Expenses:
Net losses and loss adjustment expenses	152,956	130,198
Commission expenses	37,554	32,905
Other operating expenses	60,809	54,909
Interest expense	3,858	3,855
Total expenses	255,177	221,867
Income before income taxes	32,921	38,356
Income tax expense (benefit)	9,989	12,427
Net income	$ 22,932	$ 25,929
Net income per common share:
Basic	$ 1.58	$ 1.81
Diluted	$ 1.54	$ 1.77
Average common shares outstanding:
Basic	14,492,214	14,327,606
Diluted	14,891,646	14,637,837