Employee benefit plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,968	$ 18,652	$ 18,543
Interest cost	57,348	54,970	55,873
Expected return on plan assets	(65,143)	(91,021)	(81,212)
Amortization of prior service cost / (credit)	1,022	916	910
Amortization of net actuarial gain (loss)	30,681	37,391	39,209
Effect of settlement loss	2,377	730	32,761
Other	0	256	0
Net periodic benefit cost (income)	30,253	21,894	66,084
Retiree Health and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	308	297	313
Interest cost	467	452	463
Expected return on plan assets	(718)	(1,135)	(1,636)
Amortization of prior service cost / (credit)	(498)	(498)	(499)
Amortization of net actuarial gain (loss)	(823)	(1,120)	(759)
Net periodic benefit cost (income)	$ (1,264)	$ (2,004)	$ (2,118)