Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amount [Abstract]
Dividends paid to shareholders	$ 15,803	$ 15,003	$ 14,947
Cash dividends paid (in dollars per share)	$ 1.04	$ 1.00	$ 1.00
Additional Paid-in Capital [Member]
Amount [Abstract]
Balance, Beginning period	$ 52,946	$ 51,854	$ 50,594
Restricted stock grants	1,340	1,092	1,260
Balance, Period end	$ 54,286	$ 52,946	$ 51,854
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
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
Balance, Period end	$ 112	$ 112	$ 112
Stated value of common stock (in dollars per share)	$ 0.04
Class B [Member]
Shares [Abstract]
Balance, Beginning period (in shares)	12,402,941	12,356,389	12,304,191
Restricted stock grants (in shares)	57,959	46,552	52,198
Balance, Period end (in shares)	12,460,900	12,402,941	12,356,389
Amount [Abstract]
Balance, Beginning period	$ 529	$ 527	$ 525
Restricted stock grants	3	2	2
Balance, Period end	$ 532	$ 529	$ 527
Stated value of common stock (in dollars per share)	$ 0.04