Accumulated Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments [Abstract]
Total unrealized gain before federal income taxes	$ 79,753				$ 75,521				$ 79,753	$ 75,521
Deferred tax liability	(27,913)				(26,432)				(27,913)	(26,432)
Net unrealized gains on investments	51,840				49,089				51,840	49,089
Foreign exchange adjustment [Abstract]
Gross unrealized gains	600				2,155				600	2,155
Deferred tax liability	(210)				(754)				(210)	(754)
Net unrealized gains on foreign exchange adjustment	390				1,401				390	1,401
Accumulated other comprehensive income	52,230				50,490				52,230	50,490	37,443
Investments [Abstract]
Pre-tax holding gains on debt and equity securities arising during period									12,055	36,477	15,612
Less: applicable federal income taxes									4,220	12,766	5,464
Net unrealized gains on investments									7,835	23,711	10,148
Pre-tax gains on debt and equity securities included in net income during period									7,823	15,520	1,959
Less: applicable federal income taxes									2,739	5,431	686
Other comprehensive income (loss), reclassification adjustment for sale of securities included in net income, net of tax									5,084	10,089	1,273
Change in unrealized gains on investments									2,751	13,622	8,875
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Beginning accumulated other comprehensive income				50,490				37,443	50,490	37,443	28,351
Change in foreign exchange adjustment									(1,011)	(575)	217
Change in unrealized net gains on investments									2,751	13,622	8,875
Ending accumulated other comprehensive income	52,230				50,490				52,230	50,490	37,443
Retained Earnings [Roll Forward]
Beginning retained earnings				280,003				258,358	280,003	258,358	241,325
Net income	8,246	5,770	9,340	6,361	8,967	7,771	4,907	14,943	29,717	36,588	31,919
Dividends									(14,947)	(14,943)	(14,886)
Ending retained earnings	$ 294,773				$ 280,003				$ 294,773	$ 280,003	$ 258,358