Stockholders' Equity (Details) - USD ($) $ in Thousands			12 Months Ended
	Apr. 27, 2016	Jan. 26, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Conversion of Stock [Line Items]
Additional cash issued to class B common stock holders			$ 0	$ 12,000	$ 0
Unrecognized excess tax benefits from additional paid-in capital to retained earnings					0
Number of treasury shares held			352,161	352,161
Subsidiaries
Conversion of Stock [Line Items]
Number of treasury shares held			145,820	145,820
Retained Earnings
Conversion of Stock [Line Items]
Unrecognized excess tax benefits from additional paid-in capital to retained earnings					$ 631
Conversion, Class B Common Stock to Common Stock
Conversion of Stock [Line Items]
Additional cash issued to class B common stock holders	$ 12,000	$ 12,000	$ 12,000	$ 12,000
Conversion, Class B Common Stock to Common Stock | Class B Common Stock
Conversion of Stock [Line Items]
Number of shares converted	1,050,012	1,050,012
Conversion, Class B Common Stock to Common Stock | Common Stock
Conversion of Stock [Line Items]
Number of shares issued	1,050,012	1,050,012