Pension Plan and Postretirement Benefits Other Than Pension - Components of net periodic costs, weighted average assumptions, and expected benefit payments (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jul. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Components of net periodic benefit cost:
Interest cost		$ 3,070	$ 6,146	$ 5,986
Expected return on plan assets		(2,617)	(6,315)	(8,320)
Actuarial (gain) loss			11,217	(15,501)
Settlement loss		1,302
Total		$ 1,755	$ 11,048	$ (17,835)
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		3.07%	4.45%	3.76%
Expected return on plan assets (as a percent)		2.50%	4.00%	5.00%
Expected benefit payments
Employer contributions	$ 3,700	$ 3,661	$ 0	$ 4,000
Other Postretirement Benefits
Components of net periodic benefit cost:
Interest cost		15	33	54
Actuarial gain amortization		(269)	(495)	(120)
Prior service cost amortization				(2)
Total		$ (254)	$ (462)	$ (68)
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)		2.87%	4.08%	3.28%
Expected benefit payments
2021		$ 97
2022		81
2023		58
2024		58
2025		37
2026 through 2030		110
Total		441
Employer contributions		$ 607	$ 402	$ 236