Retirement Plans - Non-contributory defined benefit pension plan - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized in accumulated other comprehensive income excluding related income tax effects
Net loss	$ (9,200,000)	$ (9,500,000)
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Net periodic pension benefit	351,000	205,000	$ 152,000
Total amount recognized in other comprehensive income	589,000	1,202,000	2,229,000
Other Comprehensive Income
Change in benefit obligation:
Service cost	127,000	112,000
Interest cost	1,124,000	1,132,000	1,017,000
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	(1,124,000)	(1,132,000)	(1,017,000)
Service cost	127,000	112,000
Expected return on plan assets	(2,213,000)	(2,131,000)	(2,067,000)
Recognized net actuarial loss	(751,000)	(816,000)	(898,000)
Net periodic pension benefit	211,000	71,000	152,000
Net (gain) loss	491,000	655,000	2,315,000
Amortization of loss	(751,000)	(816,000)	(898,000)
Total amount recognized in other comprehensive income	(260,000)	(161,000)	1,417,000
Total recognized in net periodic benefit cost and other comprehensive income	(471,000)	(232,000)	1,265,000
Non-contributory defined benefit pension plan
Change in benefit obligation:
Benefit obligation at beginning of year	28,800,000	28,260,000	28,002,000
Service cost	127,000	112,000
Interest cost	1,124,000	1,132,000	1,017,000
Actuarial (gain) loss	2,665,000	435,000	132,000
Benefits paid	(1,138,000)	(1,015,000)	(891,000)
Expenses	(78,000)	(124,000)
Benefit obligation at end of year	31,500,000	28,800,000	28,260,000
Change in plan assets:
Fair value of plan assets at beginning of year	28,216,000	27,444,000	28,451,000
Actual return on plan assets	4,387,000	1,911,000	(116,000)
Benefits paid	(1,138,000)	(1,015,000)	(891,000)
Expenses	(78,000)	(124,000)
Fair value of plan assets at end of year	31,387,000	28,216,000	27,444,000
Funded status	(113,000)	(584,000)	(816,000)
Components of net periodic pension cost and other amounts recognized in other comprehensive income
Interest cost	(1,124,000)	(1,132,000)	$ (1,017,000)
Service cost	127,000	112,000
Estimated net loss for the plan that will be amortized from other comprehensive income into periodic benefit cost over the next fiscal year
Estimated net loss for defined benefit pension plan that will be amortized from accumulated other comprehensive income into periodic benefit cost over the next fiscal year	775,000
Accumulated benefit obligation in excess of plan assets
Projected benefit obligation	31,500,000	28,800,000
Accumulated benefit obligation	31,500,000	28,800,000
Fair value of plan assets	$ 31,387,000	$ 28,216,000
Weighted-average assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.50%	4.00%
Weighted-average assumptions used to determine net periodic pension cost
Discount rate (as a percent)	4.00%	4.10%	3.70%
Expected long-term return on plan assets (as a percent)	7.75%	7.75%	7.75%
Non-contributory defined benefit pension plan | Employees hired on or before December 31, 2005
Retirement Plans
Requisite age of employees for receiving retirement benefits under the plan	21 years
Requisite age of employees for receiving retirement benefits under the new benefit formula	1 year