Pension and Other Postretirement Benefits - Periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 2,337	$ 2,453
Interest cost	3,018	3,171
Expected return on plan assets	(3,013)	(3,139)
Settlement Expense (Income)	131
Amortization of net (gain) loss	39	125
Amortization of prior service cost (credit)	44	44
Net periodic benefit cost (credit)	2,556	2,654
Postretirement Benefits
Components of net periodic benefit cost
Service cost	235	250
Interest cost	174	211
Amortization of net (gain) loss	(166)	(174)
Amortization of prior service cost (credit)	(41)	(42)
Net periodic benefit cost (credit)	$ 202	$ 245