Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 21, 2017	Aug. 31, 2017
Amount [Abstract]
Repurchase of common shares	$ (1,880)	$ 0	$ 0
Dividends paid to shareholders	$ 16,302	$ 15,803	$ 15,003
Cash dividends paid (in dollars per share)	$ 1.08	$ 1.04	$ 1.00
Stock repurchase program, shares authorized (in shares)					2,464,209
Stock repurchase program, authorized amount				$ 17,500
Repurchase of common shares amount paid	$ 1,880	$ 0	$ 0
Average share price (in dollars per share)	$ 22.12
Class A [Member]
Shares [Abstract]
Balance, Beginning period (in shares)	2,623,109
Balance, Period end (in shares)	2,623,109	2,623,109
Amount [Abstract]
Stated value of common stock (in dollars per share)	$ 0.04
Class B [Member]
Shares [Abstract]
Balance, Beginning period (in shares)	12,460,900
Balance, Period end (in shares)	12,423,518	12,460,900
Amount [Abstract]
Stated value of common stock (in dollars per share)	$ 0.04
Common Stock [Member] | Class A [Member]
Shares [Abstract]
Balance, Beginning period (in shares)	2,623,109	2,623,109	2,623,109
Restricted stock grants (in shares)	0	0	0
Repurchase of common shares (in shares)	0
Balance, Period end (in shares)	2,623,109	2,623,109	2,623,109
Amount [Abstract]
Balance, Beginning period	$ 112	$ 112	$ 112
Restricted stock grants	0	0	0
Repurchase of common shares	0
Balance, Period end	$ 112	$ 112	$ 112
Common Stock [Member] | Class B [Member]
Shares [Abstract]
Balance, Beginning period (in shares)	12,460,900	12,402,941	12,356,389
Restricted stock grants (in shares)	47,578	57,959	46,552
Repurchase of common shares (in shares)	(84,960)
Balance, Period end (in shares)	12,423,518	12,460,900	12,402,941
Amount [Abstract]
Balance, Beginning period	$ 532	$ 529	$ 527
Restricted stock grants	2	3	2
Repurchase of common shares	(4)
Balance, Period end	530	532	529
Additional Paid-in Capital [Member]
Amount [Abstract]
Balance, Beginning period	54,286	52,946	51,854
Restricted stock grants	1,152	1,340	1,092
Repurchase of common shares	(360)
Balance, Period end	$ 55,078	$ 54,286	$ 52,946