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Pensions (Details) - Pension Plans [Member] - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Components of Net Periodic Pension Cost [Abstract]          
Service cost $ 272 $ 235 $ 543 $ 469  
Interest cost 302 364 604 728  
Expected return on plan assets (913) (800) (1,826) (1,599)  
Amortization of actuarial loss 121 93 242 185  
Amortization of prior service cost (3) (3) (6) (6)  
Rate-regulated adjustment 796 686 1,593 1,373  
Net periodic pension expense 575 $ 575 1,150 $ 1,150  
Employer Contributions [Abstract]          
Estimated employer contributions in 2021         $ 2,300
Contributions made by employer     1,150    
Estimated future contributions in 2021 $ 1,150   $ 1,150