Pensions and other postretirement benefit plans - Retirement Plan Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Company's pension costs
Total non-operating (benefit) expenses	$ (23,860)	$ (18,334)	$ (71,644)	$ (54,433)
Pension Benefits
Company's pension costs
Service cost - benefits earned during the period	490	693	1,470	2,078
Interest cost on benefit obligation	17,042	20,670	51,179	61,803
Expected return on plan assets	(41,355)	(39,573)	(124,193)	(117,940)
Amortization of actuarial loss (gain)	38	(26)	115	(80)
Total non-operating (benefit) expenses	(24,275)	(18,929)	(72,899)	(56,217)
Total expense (benefit) for retirement plans	(23,785)	(18,236)	(71,429)	(54,139)
Postretirement Benefits
Company's pension costs
Service cost - benefits earned during the period	23	27	67	79
Interest cost on benefit obligation	428	580	1,330	1,740
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss (gain)	(13)	15	(75)	44
Total non-operating (benefit) expenses	415	595	1,255	1,784
Total expense (benefit) for retirement plans	$ 438	$ 622	$ 1,322	$ 1,863