Pensions and other postretirement benefit plans - Retirement Plan Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Company's pension costs
Total non-operating (benefit) expenses	$ (14,990)	$ (23,860)	$ (51,363)	$ (71,644)
Pension benefits
Company's pension costs
Service cost - benefits earned during the period	448	490	1,357	1,470
Interest cost on benefit obligation	17,715	17,042	54,496	51,179
Expected return on plan assets	(32,316)	(41,355)	(106,106)	(124,193)
Amortization of actuarial loss (gain)	22	38	67	115
Pension settlement gain	(706)	0	(706)	0
Total non-operating (benefit) expenses	(15,285)	(24,275)	(52,249)	(72,899)
Total expense (benefit) for retirement plans	(14,837)	(23,785)	(50,892)	(71,429)
Postretirement benefits
Company's pension costs
Service cost - benefits earned during the period	19	23	57	67
Interest cost on benefit obligation	442	428	1,327	1,330
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss (gain)	(147)	(13)	(441)	(75)
Total non-operating (benefit) expenses	295	415	886	1,255
Total expense (benefit) for retirement plans	$ 314	$ 438	$ 943	$ 1,322