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Retirement plans - Funded Status of Principal Retirement Plans (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets $ 407,550    
Benefit Obligation 554,795    
Funded status at end of year (147,245)    
Retirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 407,550 $ 439,149 $ 388,168
Benefit Obligation 554,795 614,111 $ 606,413
Funded status at end of year (147,245) $ (174,962)  
SERP      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 0    
Benefit Obligation 62,892    
Funded status at end of year (62,892)    
All other      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 0    
Benefit Obligation 549    
Funded status at end of year (549)    
TRP | Retirement Plans      
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]      
Fair Value of Plan Assets 407,550    
Benefit Obligation 491,354    
Funded status at end of year $ (83,804)