Retirement plans - Funded Status of Principal Retirement Plans (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	$ 552,996
Benefit Obligation	646,662
Funded status as of end of year	(93,666)
Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	552,996	$ 479,735	$ 407,550
Benefit Obligation	646,662	613,695	$ 554,795
Funded status as of end of year	(93,666)	$ (133,960)
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	0
Benefit Obligation	69,294
Funded status as of end of year	(69,294)
All other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	0
Benefit Obligation	522
Funded status as of end of year	(522)
TRP | Retirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	552,996
Benefit Obligation	576,846
Funded status as of end of year	$ (23,850)