Stockholders' Equity - Components of stockholders' equity and accumulated other comprehensive loss (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jul. 28, 2004
Changes in the components of stockholders' equity
Balance at December 31, 2017	$ 59,222,000		$ 58,073,000		$ 59,222,000
Balance at the beginning of the period	59,222,000
Net loss	(992,000)	$ (2,405,000)
Change in net actuarial loss and prior service cost, net of income tax expense of $0	(27,000)	$ (25,000)
Balance at the end of the period	58,073,000
Income tax expense on change in net actuarial loss and prior service cost	$ 10,000
Accumulated other comprehensive loss, net of income taxes
Net actuarial loss and prior service cost not yet recognized in net periodic benefit cost, net of income tax benefit of $000 and $2,410,000, respectively			(3,486,000)		(3,513,000)
Accumulated unrealized gain on available-for-sale securities, net of income tax expense of $52,000					73,000
Accumulated other comprehensive loss			(3,486,000)		(3,440,000)
Income tax benefit on net actuarial loss and prior service cost not yet recognized in net periodic benefit cost			$ 2,400,000		2,410,000
Income tax expense on accumulated unrealized gain on available-for-sale securities					52,000
Number of shares purchased and retired	47,236	46,179
Average purchase price of shares purchased and retired (in dollars per share)	$ 2.00	$ 2.27
Share Repurchase Authorization 2004
Accumulated other comprehensive loss, net of income taxes
Number of shares purchased and retired	92,479	0
Average purchase price of shares purchased and retired (in dollars per share)	$ 2.06
Remaining number of shares authorized to be repurchased			917,098
Share Repurchase Authorization 2004 | Maximum
Accumulated other comprehensive loss, net of income taxes
Number of shares of common stock authorized to be repurchased						2,000,000
Common Stock | Common Stock
Changes in the components of stockholders' equity
Balance at December 31, 2017	$ 1,826,000		$ 1,826,000
Issuance of restricted stock awards, net of forfeitures	15,000
Repurchase and retirement of common stock	(14,000)
Balance at the end of the period	1,826,000
Common Stock | Common Stock | Previously reported
Changes in the components of stockholders' equity
Balance at December 31, 2017	1,825,000				1,825,000
Balance at the beginning of the period	1,825,000
Common Stock | Class A Common Stock
Changes in the components of stockholders' equity
Balance at December 31, 2017	1,851,000		1,851,000
Balance at the end of the period	1,851,000
Common Stock | Class A Common Stock | Previously reported
Changes in the components of stockholders' equity
Balance at December 31, 2017	1,851,000				1,851,000
Balance at the beginning of the period	1,851,000
Additional Paid-in Capital
Changes in the components of stockholders' equity
Balance at December 31, 2017	101,659,000		101,659,000
Issuance of restricted stock awards, net of forfeitures	(15,000)
Stock-based compensation	105,000
Repurchase and retirement of common stock	(275,000)
Balance at the end of the period	101,659,000
Additional Paid-in Capital | Previously reported
Changes in the components of stockholders' equity
Balance at December 31, 2017	101,844,000				101,844,000
Balance at the beginning of the period	101,844,000
Accumulated Deficit
Changes in the components of stockholders' equity
Balance at December 31, 2017	(43,777,000)		(43,777,000)
Net loss	(992,000)
Balance at the end of the period	(43,777,000)
Accumulated Deficit | Previously reported
Changes in the components of stockholders' equity
Balance at December 31, 2017	(42,858,000)				(42,858,000)
Balance at the beginning of the period	(42,858,000)
Accumulated Deficit | ASU 2016-01 | Adjustment
Changes in the components of stockholders' equity
Adoption of ASU 2016-01 (see NOTE 3)				$ 73,000	73,000
Accumulated Other Comprehensive Loss
Changes in the components of stockholders' equity
Balance at December 31, 2017	(3,486,000)		$ (3,486,000)
Change in net actuarial loss and prior service cost, net of income tax expense of $0	27,000
Balance at the end of the period	(3,486,000)
Accumulated Other Comprehensive Loss | Previously reported
Changes in the components of stockholders' equity
Balance at December 31, 2017	(3,440,000)				(3,440,000)
Balance at the beginning of the period	$ (3,440,000)
Accumulated Other Comprehensive Loss | ASU 2016-01 | Adjustment
Changes in the components of stockholders' equity
Adoption of ASU 2016-01 (see NOTE 3)				$ (73,000)	$ (73,000)