Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investments [Abstract]
Total unrealized gain before federal income taxes	$ 59,883				$ 79,753				$ 59,883	$ 79,753
Deferred tax liability	(20,959)				(27,913)				(20,959)	(27,913)
Net unrealized gains on investments	38,924				51,840				38,924	51,840
Foreign exchange adjustment [Abstract]
Total unrealized gains (losses)	(1,640)				600				(1,640)	600
Deferred tax benefit (liability)	574				(210)				574	(210)
Net unrealized gains (losses) on foreign exchange adjustment	(1,066)				390				(1,066)	390
Accumulated other comprehensive income	37,858				52,230				37,858	52,230
Investments [Abstract]
Pre-tax holding gains (losses) on debt and equity securities arising during period									(19,445)	12,055	$ 36,477
Less: applicable federal income taxes									(6,806)	4,220	12,766
Net unrealized gains on investments									(12,639)	7,835	23,711
Pre-tax gains on debt and equity securities included in net income during period									426	7,823	15,520
Less: applicable federal income taxes									149	2,739	5,431
Other comprehensive income (loss), reclassification adjustment for sale of securities included in net income, net of tax									277	5,084	10,089
Change in unrealized gains on investments									(12,916)	2,751	13,622
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year				$ 399,496				$ 381,724	399,496	381,724	346,712
Change in unrealized net gains on investments									(12,916)	2,751	13,622
Shareholders' equity at end of year:	394,498				399,496				394,498	399,496	381,724
Retained Earnings [Roll Forward]
Beginning retained earnings				294,773				280,003	294,773	280,003	258,358
Net income	3,541	$ 7,780	$ 5,718	6,243	8,246	$ 5,770	$ 9,340	6,361	23,283	29,717	36,588
Dividends									(15,003)	(14,947)	(14,943)
Ending retained earnings	303,053				294,773				303,053	294,773	280,003
AOCI Attributable to Parent [Member]
Investments [Abstract]
Change in unrealized gains on investments									(12,916)	2,751	13,622
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year				$ 52,230				$ 50,490	52,230	50,490	37,443
Change in foreign exchange adjustment									(1,456)	(1,011)	(575)
Change in unrealized net gains on investments									(12,916)	2,751	13,622
Shareholders' equity at end of year:	$ 37,858				$ 52,230				$ 37,858	$ 52,230	$ 50,490