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Employee benefit plans (Details) - USD ($)
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Qualified plan | Underfunded plan | Pension benefits    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Interest cost $ 3,789,000 $ 2,631,000
Expected return on assets (4,749,000) (4,864,000)
Amortization of net actuarial (gain) loss 901,000 1,671,000
Net periodic benefit cost (credit) (59,000) (562,000)
Qualified plan | Overfunded Plan | Other postretirement benefits    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Service cost 227,000 354,000
Interest cost 670,000 474,000
Expected return on assets (1,341,000) (1,322,000)
Amortization of prior service credit (350,000) (350,000)
Amortization of net actuarial (gain) loss (213,000) (54,000)
Net periodic benefit credit, including amount capitalized (1,007,000) (898,000)
Less amount capitalized 24,000 31,000
Net periodic benefit cost (credit) (1,031,000) (929,000)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans    
Defined benefit plan disclosure, net periodic benefit cost [Line Items]    
Net periodic benefit cost (credit) $ 936,000 $ 773,000