Stockholders' Equity - Accumulated other comprehensive loss (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Accumulated other comprehensive loss
Net actuarial loss and prior service cost not yet recognized in net periodic benefit cost, net of income tax benefit of $3,152,000 and $3,080,000, respectively	$ (4,733,000)	$ (4,624,000)
Accumulated unrealized gain on available-for-sale securities, net of income tax expense of $23,000 and $19,000, respectively	34,000	28,000
Accumulated other comprehensive loss	(4,699,000)	(4,596,000)
Income tax benefit of net actuarial loss and prior service cost not yet recognized in net periodic benefit cost	3,152,000	3,080,000
Income tax expense of accumulated unrealized gain on available-for-sale securities	$ 23,000	$ 19,000