Pension Plans - SERP (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
SERP
Deferred contribution plan
Expenses recorded under the plan	$ 20,000	$ 20,000	$ 40,000	$ 40,000
Employer contributions	0	0	96,000	81,000
Liability for pension benefits	41,000		41,000		$ 97,000
401(k) plan
Deferred contribution plan
Expenses recorded under the plan	$ 29,000	$ 29,000	$ 62,000	$ 67,000