Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Amount [Abstract]
Restricted stock grants			$ 901	$ 920
Repurchase of common shares			(1,280)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance			418,811	404,345
Net income	$ 2,487	$ (12,343)	2,817	(5,588)
Other comprehensive income (loss)	(2,214)	3,868	(5,558)	9,268
Cash dividends paid to shareholders			(8,456)	(8,174)
Restricted stock grants			901	920
Repurchase of common shares			(1,280)
Ending balance	$ 407,235	$ 400,771	$ 407,235	$ 400,771
Class A [Member]
Shares [Abstract]
Balance, Beginning period (in shares)			2,623,109
Balance, Period end (in shares)	2,622,809		2,622,809
Class B [Member]
Shares [Abstract]
Balance, Beginning period (in shares)			12,423,518
Balance, Period end (in shares)	12,379,561		12,379,561
Common Stock [Member] | Class A [Member]
Shares [Abstract]
Balance, Beginning period (in shares)			2,623,109
Restricted stock grants (in shares)			0
Repurchase of common shares (in shares)			(300)
Balance, Period end (in shares)	2,622,809		2,622,809
Amount [Abstract]
Balance, Beginning period			$ 112
Restricted stock grants			0
Repurchase of common shares			0
Balance, Period end	$ 112		112
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted stock grants			0
Repurchase of common shares			$ 0
Common Stock [Member] | Class B [Member]
Shares [Abstract]
Balance, Beginning period (in shares)			12,423,518
Restricted stock grants (in shares)			10,128
Repurchase of common shares (in shares)			(54,085)
Balance, Period end (in shares)	12,379,561		12,379,561
Amount [Abstract]
Balance, Beginning period			$ 530
Restricted stock grants			0
Repurchase of common shares			(2)
Balance, Period end	$ 528		528
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted stock grants			0
Repurchase of common shares			(2)
Additional Paid-in Capital [Member]
Amount [Abstract]
Balance, Beginning period			55,078
Restricted stock grants			901
Repurchase of common shares			(234)
Balance, Period end	$ 55,745		55,745
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Restricted stock grants			901
Repurchase of common shares			$ (234)