Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments [Abstract]
Total unrealized gain before federal income taxes	$ 52,386				$ 59,883				$ 52,386	$ 59,883
Deferred tax liability	(18,335)				(20,959)				(18,335)	(20,959)
Net unrealized gains on investments	34,051				38,924				34,051	38,924
Foreign exchange adjustment [Abstract]
Total unrealized losses	(1,278)				(1,640)				(1,278)	(1,640)
Deferred tax benefit	447				574				447	574
Net unrealized losses on foreign exchange adjustment	(831)				(1,066)				(831)	(1,066)
Accumulated other comprehensive income	33,220				37,858				33,220	37,858
Investments [Abstract]
Pre-tax holding gains (losses) on debt and equity securities arising during period									13,259	(19,445)	$ 12,055
Less: applicable federal income taxes									4,641	(6,806)	4,220
Net unrealized gains on investments									8,618	(12,639)	7,835
Pre-tax gains on debt and equity securities included in net income during period									20,755	426	7,823
Less: applicable federal income taxes									7,264	149	2,739
Other comprehensive income (loss), reclassification adjustment for sale of securities included in net income, net of tax									13,491	277	5,084
Change in unrealized gains (losses) on investments									(4,873)	(12,916)	2,751
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year				$ 394,498				$ 399,496	394,498	399,496	381,724
Change in foreign exchange adjustment									235	(1,456)	(1,011)
Change in unrealized net gains on investments									(4,873)	(12,916)	2,751
Shareholders' equity at end of year:	404,345				394,498				404,345	394,498	399,496
Retained Earnings [Roll Forward]
Beginning retained earnings				303,053				294,773	303,053	294,773	280,003
Net income	4,864	$ 4,001	$ 5,969	14,112	3,541	$ 7,780	$ 5,718	6,243	28,945	23,283	29,717
Dividends									(15,803)	(15,003)	(14,947)
Ending retained earnings	316,195				303,053				316,195	303,053	294,773
AOCI Attributable to Parent [Member]
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year				$ 37,858				$ 52,230	37,858	52,230	50,490
Shareholders' equity at end of year:	$ 33,220				$ 37,858				$ 33,220	$ 37,858	$ 52,230