Pension Benefits (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Net periodic benefit cost:
Service cost	$ 425,000	$ 600,000	$ 850,000	$ 1,200,000
Interest cost	2,066,000	2,242,000	4,131,000	4,484,000
Expected return on plan assets	(3,210,000)	(3,117,000)	(6,421,000)	(6,233,000)
Amortization of net actuarial loss	210,000	247,000	421,000	494,000
Total net periodic benefit	$ (509,000)	$ (28,000)	$ (1,019,000)	$ (55,000)
Weighted average assumptions used to determine net cost
Discount rate	3.90%	4.10%	3.90%	4.10%
Expected return on plan assets	7.25%	7.25%	7.25%	7.25%
Employer contributions			$ 1,500,000	$ 3,000,000