Pension and Other Postretirement Benefits - Periodic benefit cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 8,839	$ 9,067	$ 9,217
Interest cost	10,937	11,897	11,832
Expected return on plan assets	(12,987)	(13,041)	(12,006)
Curtailment income	(118)	(97)	0
Settlement expense	4,324	4,982	2,740
Amortization of net (gain) loss	1,530	1,687	804
Amortization of prior service cost (credit)	247	175	175
Net periodic benefit cost (credit)	12,772	14,670	12,762
Postretirement Benefits
Components of net periodic benefit cost
Service cost	574	657	955
Interest cost	557	490	713
Expected return on plan assets	0	0	0
Curtailment income	0	0	0
Settlement expense	0	0	0
Amortization of net (gain) loss	(1,436)	(1,540)	(601)
Amortization of prior service cost (credit)	(137)	(137)	(137)
Net periodic benefit cost (credit)	$ (442)	$ (530)	$ 930