Pension and Other Postretirement Benefits - Periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 2,259	$ 2,184	$ 4,702	$ 4,521
Interest cost	2,989	2,934	5,947	5,952
Expected return on plan assets	(3,198)	(2,934)	(6,477)	(5,947)
Settlement expense	464	185	472	316
Amortization of net (gain) loss	732	258	1,252	297
Amortization of prior service cost (credit)	43	43	87	87
Net periodic benefit cost (income)	3,289	2,670	5,983	5,226
Postretirement Benefits
Components of net periodic benefit cost
Service cost	154	243	329	478
Interest cost	118	183	245	357
Expected return on plan assets	0	0	0	0
Settlement expense	0	0	0	0
Amortization of net (gain) loss	(419)	(135)	(770)	(301)
Amortization of prior service cost (credit)	(34)	(27)	(68)	(68)
Net periodic benefit cost (income)	$ (181)	$ 264	$ (264)	$ 466