Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Total unrealized gain before federal income taxes	$ 71,848	$ 52,386
Deferred tax liability	(25,148)	(18,335)
Net unrealized gains on investments	46,700	34,051
Foreign exchange adjustment [Abstract]
Total unrealized losses	(475)	(1,278)
Deferred tax benefit	166	447
Net unrealized losses on foreign exchange adjustment	(309)	(831)
Accumulated other comprehensive income	46,391	33,220
Investments [Abstract]
Pre-tax holding gains (losses) on debt and equity securities arising during period	26,677	13,259	$ (19,445)
Less: applicable federal income taxes	9,337	4,641	(6,806)
Net unrealized gains (losses) on investments	17,340	8,618	(12,639)
Pre-tax gains on debt and equity securities included in net income during period	7,217	20,755	426
Less: applicable federal income taxes	2,526	7,264	149
Other comprehensive income (loss), reclassification adjustment for sale of securities included in net income, net of tax	4,691	13,491	277
Change in unrealized gains (losses) on investments	12,649	(4,873)	(12,916)
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year:	404,345	394,498	399,496
Change in foreign exchange adjustment	522	235	(1,456)
Change in unrealized net gains on investments	12,649	(4,873)	(12,916)
Shareholders' equity at end of year:	418,811	404,345	394,498
AOCI Attributable to Parent [Member]
Reconciliation of Accumulated Other Comprehensive Income [Roll Forward]
Shareholders' equity at beginning of year:	33,220	37,858	52,230
Shareholders' equity at end of year:	$ 46,391	$ 33,220	$ 37,858