Pensions and other postretirement benefit plans - Retirement Plan Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 29, 2019	Sep. 30, 2020	Sep. 29, 2019
Pension
Company's pension costs
Service cost - Benefits earned during the period	$ 693	$ 159	$ 2,078	$ 476
Interest cost on benefit obligation	20,670	737	61,803	2,210
Expected return on plan assets	(39,573)	(937)	(117,940)	(2,812)
Amortization of actuarial loss (gain)	(26)	(39)	(80)	(117)
Total non-operating expenses (benefit)	(18,929)	(239)	(56,217)	(719)
Total expense (benefit) for retirement plans	(18,236)	(80)	(54,139)	(243)
OPEB
Company's pension costs
Service cost - Benefits earned during the period	27	0	79	0
Interest cost on benefit obligation	580	22	1,740	67
Expected return on plan assets	0	0	0	0
Amortization of actuarial loss (gain)	15	9	44	27
Total non-operating expenses (benefit)	595	31	1,784	94
Total expense (benefit) for retirement plans	$ 622	$ 31	$ 1,863	$ 94