Consolidated Statements of Income (Loss) (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Gross Written Premiums	$ 497,236	$ 452,179	$ 992,460	$ 902,484
Revenues:
Net Written Premiums	379,292	333,282	772,554	670,445
Change in Unearned Premiums	(48,277)	(39,447)	(118,912)	(90,479)
Net Earned Premiums	331,015	293,835	653,642	579,966
Net Investment Income	24,601	22,265	48,303	43,713
Net Realized and Unrealized Gains (Losses):
Total Other-Than-Temporary Impairment Losses	(18)	29	(55)	(1,048)
Portion of Loss Recognized in Other Comprehensive Income (Before Tax)	18	(29)	55	(45)
Net Other-Than-Temporary Impairment Losses Recognized in Earnings	0	0	0	(1,093)
Net Realized Gains on Investments Sold	1,787	1,694	2,956	2,743
Net Unrealized Gains (Losses) on Equity Securities at Fair Value	1,329		(1,852)
Total Net Realized and Unrealized Gains	3,116	1,694	1,104	1,650
Other Income (Loss)	2,628	(411)	2,511	657
Total Revenues	361,360	317,383	705,560	625,986
Expenses:
Net Losses and Loss Adjustment Expenses	196,333	177,110	382,478	346,710
Commission Expenses	53,193	48,173	107,345	96,017
Other Operating Expenses	68,182	60,766	131,108	119,304
Interest Expense	3,864	3,861	7,728	7,722
Total Expenses	321,572	289,910	628,659	569,753
Income (Loss) Before Income Taxes	39,788	27,473	76,901	56,233
Income Tax Expense	7,684	6,971	13,919	14,621
Net Income (Loss)	$ 32,104	$ 20,502	$ 62,982	$ 41,612
Net Income per Common Share:
Basic	$ 1.08	$ 0.70	$ 2.12	$ 1.42
Diluted	$ 1.07	$ 0.69	$ 2.09	$ 1.39
Average common shares outstanding:
Basic	29,733	29,470	29,664	29,377
Diluted	30,103	29,918	30,143	29,897
Cash Dividends Declared per Common Share	$ 0.07	$ 0.06	$ 0.14	$ 0.105