Pension Plans - Components of net periodic pension benefit (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Components of net periodic pension benefit
Interest cost	$ 206,000	$ 217,000
Expected return on plan assets	(301,000)	(285,000)
Recognized net actuarial loss	51,000	34,000
Total net periodic pension cost	(44,000)	(34,000)
Employer contribution	50,000	$ 40,000
Expected additional contributions during the remainder of 2017	$ 240,000