Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Sep. 24, 2018	Aug. 07, 2018	Aug. 31, 2017
Increase (Decrease) in Common Stock Outstanding and Additional Paid-In Capital [Roll Forward]
Dividends paid to shareholders	$ 16,835		$ 16,302	$ 15,803
Cash dividends paid (in dollars per share)	$ 1.12		$ 1.08	$ 1.04
Stock repurchase program, shares authorized (in shares)							2,464,209
Repurchase of common shares amount paid	$ 4,596		$ 1,880	$ 0
Investments [Abstract]
Total unrealized gain (loss) before federal income tax expense (benefit)	(7,859)		71,848
Deferred tax benefit (liability)	1,651		(25,148)
Net unrealized gains (losses) on investments	(6,208)		46,700
Foreign exchange adjustment [Abstract]
Total unrealized losses	(1,442)		(475)
Deferred tax benefit	303		166
Net unrealized losses on foreign exchange adjustment	(1,139)		(309)
Accumulated other comprehensive income (loss)	(7,347)		46,391
Investments [Abstract]
Pre-tax holding gains (losses) on debt and equity securities arising during period	(12,253)	[1]	26,677	13,259
Less: applicable federal income tax expense (benefit)	(2,573)		9,337	4,641
Net unrealized gains (losses) on investments	(9,680)		17,340	8,618
Pre-tax gains (losses) on debt and equity securities included in net income (loss) during period	(3,560)	[1]	7,217	20,755
Less: applicable federal income tax expense (benefit)	(748)		2,526	7,264
Other comprehensive income (loss), reclassification adjustment for sale of securities included in net income, net of tax	(2,812)		4,691	13,491
Change in unrealized gains (losses) on investments	$ (6,868)		$ 12,649	$ (4,873)
Maximum [Member]
Increase (Decrease) in Common Stock Outstanding and Additional Paid-In Capital [Roll Forward]
Stock repurchase program, authorized amount					$ 12,000	$ 25,000
Class A [Member]
Increase (Decrease) in Common Stock Outstanding and Additional Paid-In Capital [Roll Forward]
Stated value of common stock (in dollars per share)	$ 0.04
Repurchase of common stock (in shares)	7,770
Average share price (in dollars per share)	$ 21.58
Class B [Member]
Increase (Decrease) in Common Stock Outstanding and Additional Paid-In Capital [Roll Forward]
Stated value of common stock (in dollars per share)	$ 0.04
Repurchase of common stock (in shares)	191,898
Average share price (in dollars per share)	$ 23.07

[1] Effective January 1, 2018, the Company adopted ASU 2016-01 and unrealized gains (losses) related to equity securities are no longer reflected in accumulated other comprehensive income (loss). Prior periods have not been restated to conform to the current presentation.