Pension and Other Postretirement Benefits - Periodic benefit cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 2,184	$ 2,456	$ 4,521	$ 4,909
Interest cost	2,934	3,122	5,952	6,293
Expected return on plan assets	(2,934)	(3,067)	(5,947)	(6,206)
Settlement expense	185		316
Amortization of net (gain) loss	258	122	297	247
Amortization of prior service cost (credit)	43	43	87	87
Net periodic benefit cost	2,670	2,676	5,226	5,330
Postretirement Benefits
Components of net periodic benefit cost
Service cost	243	250	478	500
Interest cost	183	212	357	423
Amortization of net (gain) loss	(135)	(174)	(301)	(348)
Amortization of prior service cost (credit)	(27)	(42)	(68)	(84)
Net periodic benefit cost	$ 264	$ 246	$ 466	$ 491