Pension Plans - Defined benefit pension plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic pension benefit
Interest cost	$ 109,000	$ 114,000	$ 218,000	$ 229,000
Expected return on plan assets	(164,000)	(155,000)	(327,000)	(311,000)
Recognized net actuarial loss	42,000	30,000	84,000	59,000
Net periodic pension benefit	(13,000)	(11,000)	(25,000)	(23,000)
Contributions to pension plans	50,000	$ 75,000	50,000	$ 75,000
Contribution to remainder of 2017	$ 125,000		$ 125,000