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Employee benefit plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Qualified plan | Underfunded plan | Pension benefits        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost $ 0 $ 0 $ 0 $ 0
Interest cost 2,455 3,023 7,365 9,070
Expected return on assets (4,894) (4,987) (14,682) (14,962)
Amortization of prior service credit 0 0 0 0
Amortization of net actuarial loss 2,004 1,793 6,012 5,379
Net periodic benefit credit, including amount capitalized (435) (171) (1,305) (513)
Less amount capitalized 0 0 0 0
Net periodic benefit cost (credit) (435) (171) (1,305) (513)
Qualified plan | Overfunded Plan | Other postretirement benefits        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Service cost 400 383 1,200 1,149
Interest cost 466 609 1,398 1,827
Expected return on assets (1,275) (1,115) (3,825) (3,764)
Amortization of prior service credit (349) (349) (1,047) (1,048)
Amortization of net actuarial loss 6 71 18 215
Net periodic benefit credit, including amount capitalized (752) (401) (2,256) (1,621)
Less amount capitalized 46 39 129 106
Net periodic benefit cost (credit) (798) (440) (2,385) (1,727)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans        
Defined benefit plan disclosure, net periodic benefit cost [Line Items]        
Net periodic benefit cost (credit) $ 769 $ 1,000 $ 2,300 $ 2,900