Employee benefit plans - Amounts Recognized in Other Comprehensive Loss/(Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Plan
Adjustments arising during the period:
Net actuarial loss/(gain)	$ 146,414	$ 58,544	$ (10,732)
Prior service cost/(credit)	1,667	2,906	639
Net settlements/curtailments	(2,377)	(986)	(32,761)
Reversal of amortization:
Net actuarial (loss)/gain	(30,681)	(37,391)	(39,209)
Prior service (cost)/credit	(1,022)	(916)	(910)
Total recognized in other comprehensive loss/(income)	114,001	22,157	(82,973)
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	144,254	44,051	(16,889)
Retiree Health and Life Insurance Plans
Adjustments arising during the period:
Net actuarial loss/(gain)	(914)	1,738	(3,525)
Prior service cost/(credit)	0	0	0
Net settlements/curtailments	0	0	0
Reversal of amortization:
Net actuarial (loss)/gain	823	1,120	759
Prior service (cost)/credit	498	498	499
Total recognized in other comprehensive loss/(income)	407	3,356	(2,267)
Total recognized in net periodic benefit cost and other comprehensive loss/(income)	$ (857)	$ 1,352	$ (4,385)