Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 3,380	$ 2,349	$ 10,140	$ 7,047
Expected return on assets	(4,299)	(4,370)	(12,897)	(13,112)
Amortization of net actuarial (gain) loss	773	1,456	2,319	4,370
Net periodic benefit cost (credit)	(146)	(565)	(438)	(1,695)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	131	223	405	669
Interest cost	489	346	1,467	1,038
Expected return on assets	(1,334)	(1,319)	(4,026)	(3,957)
Amortization of prior service credit	(330)	(330)	(989)	(989)
Amortization of net actuarial (gain) loss	(96)	(142)	(408)	(425)
Net periodic benefit credit, including amount capitalized	(1,140)	(1,222)	(3,551)	(3,664)
Less amount capitalized	23	47	76	131
Net periodic benefit cost (credit)	(1,163)	(1,269)	(3,627)	(3,795)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 759	$ 648	$ 2,300	$ 1,900