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Retirement plans - Funded Status of Principal Retirement Plans (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets $ 479,735    
Benefit Obligation 613,695    
Funded status as of end of year (133,960)    
Retirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 479,735 $ 407,550 $ 439,149
Benefit Obligation 613,695 554,795 $ 614,111
Funded status as of end of year (133,960) $ (147,245)  
SERP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 0    
Benefit Obligation 66,085    
Funded status as of end of year (66,085)    
All other      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 0    
Benefit Obligation 470    
Funded status as of end of year (470)    
TRP | Retirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 479,735    
Benefit Obligation 547,140    
Funded status as of end of year $ (67,405)