Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Gross Written Premiums	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 367,539	$ 374,930	$ 412,565	$ 413,877	$ 1,713,265	$ 1,568,911	$ 1,453,502
Revenues:
Net Written Premiums	304,869	296,016	333,282	337,163	282,868	277,001	306,535	319,820	1,271,330	1,186,224	1,043,860
Change in Net Unearned Premiums	230	5,339	(39,447)	(51,032)	1,117	7,009	(38,543)	(55,462)	(84,910)	(85,879)	(59,773)
Net Earned Premiums	305,099	301,355	293,835	286,131	283,985	284,010	267,992	264,358	1,186,420	1,100,345	984,087
Net Investment Income	22,982	22,598	22,265	21,448	20,107	19,875	19,875	19,594	89,293	79,451	68,718
Total Other-Than-Temporary Impairment Losses	3	(957)	29	(1,077)	21	23	(162)	(109)	(2,002)	(227)	(1,870)
Portion of Loss Recognized in Other Comprehensive Income (Before Tax)	(3)	(15)	(29)	(16)	(21)	(23)	12	109	(62)	77	172
Net Other-Than-Temporary Impairment Losses Recognized In Earnings									(2,064)	(150)	(1,698)
Net Realized Gains	37,141	5,190	1,694	1,049	4,043	1,586	1,960	1,597	45,073	9,186	8,373
Other Income (Loss)	(3,201)	(1,699)	(411)	1,068	1,905	(183)	4,430	2,549	(4,243)	8,701	(491)
Total Revenues	362,021	326,472	317,383	308,603	310,040	305,288	294,107	288,098	1,314,479	1,197,533	1,058,989
Expenses:
Net Losses and Loss Adjustment Expenses	183,384	276,171	177,110	169,600	172,493	172,793	167,206	152,956	806,265	665,448	572,598
Commission Expenses	43,205	45,509	48,173	47,844	44,154	42,611	40,726	37,554	184,731	165,045	129,977
Other Operating Expenses	68,153	45,773	60,766	58,538	58,076	56,137	59,074	60,809	233,230	234,096	223,516
Interest Expense	3,863	3,862	3,861	3,861	3,860	3,859	3,858	3,858	15,447	15,435	15,424
Total Expenses	298,605	371,315	289,910	279,843	278,583	275,400	270,864	255,177	1,239,673	1,080,024	941,515
Income (Loss) Before Income Taxes	63,416	(44,843)	27,473	28,760	31,457	29,888	23,243	32,921	74,806	117,509	117,474
Income Tax Expense	36,555	(16,864)	6,971	7,650	9,866	7,875	7,053	9,989	34,312	34,783	36,417
Net Income (Loss)	$ 26,861	$ (27,979)	$ 20,502	$ 21,110	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 40,494	$ 82,726	$ 81,057
Net Income Per Common Share:
Basic	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 0.74	$ 0.76	$ 0.56	$ 0.79	$ 1.38	$ 2.85	$ 2.82
Diluted	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 0.71	$ 0.73	$ 0.54	$ 0.77	$ 1.35	$ 2.75	$ 2.73
Average Common Shares Outstanding:
Basic									29,441	29,074	28,785
Diluted									30,071	30,032	29,651