Pension and Other Postretirement Benefits - Periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 9,217	$ 9,763	$ 15,683
Interest cost	11,832	12,356	12,338
Expected return on plan assets	(12,006)	(12,189)	(11,372)
Curtailment Expense (Income)			(2,421)
Settlement expense	2,740	1,148
Amortization of net (gain) loss	804	471	1,152
Amortization of prior service cost (credit)	175	175	22
Net periodic benefit cost	12,762	11,724	15,402
Postretirement Benefits
Components of net periodic benefit cost
Service cost	955	888	1,060
Interest cost	713	779	787
Amortization of net (gain) loss	(601)	(912)	(490)
Amortization of prior service cost (credit)	(137)	(163)	(168)
Net periodic benefit cost	$ 930	$ 592	$ 1,189