Pension Plan and Postretirement Benefits Other Than Pension - Components of net periodic costs, weighted average assumptions, and expected benefit payments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit cost:
Curtailment gain				$ (31,600)
Pension Benefits
Components of net periodic benefit cost:
Service cost				8,367
Interest cost		$ 6,146	$ 5,986	6,248
Expected return on plan assets		(6,315)	(8,320)	(10,113)
Actuarial (gain) loss		11,217	(15,501)	14,091
Curtailment gain				(31,621)
Total		$ 11,048	$ (17,835)	$ (13,028)
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		4.45%	3.76%
Expected return on plan assets (as a percent)	6.00%	4.00%	5.00%
Rate of compensation increase (as a percent)				5.12%
Expected benefit payments
2020		$ 8,947
2021		9,491
2022		9,164
2023		9,469
2024		9,770
2025 through 2029		50,283
Total		97,124
Pension Benefits | Maximum
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)				4.39%
Expected return on plan assets (as a percent)				7.00%
Pension Benefits | Minimum
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)				3.96%
Expected return on plan assets (as a percent)				6.00%
Other Postretirement Benefits
Components of net periodic benefit cost:
Interest cost		33	$ 54	$ 58
Actuarial gain amortization		(495)	(120)	(180)
Prior service cost amortization			(2)	(4)
Total		$ (462)	$ (68)	$ (126)
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		4.08%	3.28%	3.46%
Expected benefit payments
2020		$ 158
2021		116
2022		99
2023		66
2024		66
2025 through 2029		143
Total		648
Contributions by participants		$ 275	$ 366	$ 506