Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 3,380	$ 2,349	$ 7,169	$ 4,698
Expected return on assets	(4,299)	(4,371)	(9,048)	(8,742)
Amortization of net actuarial (gain) loss	773	1,457	1,674	2,914
Net periodic benefit cost (credit)	(146)	(565)	(205)	(1,130)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	136	223	274	446
Interest cost	489	346	978	692
Expected return on assets	(1,334)	(1,319)	(2,668)	(2,638)
Amortization of prior service credit	(329)	(330)	(659)	(660)
Amortization of net actuarial (gain) loss	(70)	(142)	(126)	(284)
Net periodic benefit credit, including amount capitalized	(1,108)	(1,222)	(2,201)	(2,444)
Less amount capitalized	53	53	77	84
Net periodic benefit cost (credit)	(1,161)	(1,275)	(2,278)	(2,528)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 750	$ 648	$ 1,500	$ 1,300