Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 2,631	$ 2,455	$ 5,262	$ 4,910
Expected return on assets	(4,864)	(4,894)	(9,728)	(9,788)
Amortization of net actuarial (gain) loss	1,671	2,004	3,342	4,008
Net periodic benefit cost (credit)	(562)	(435)	(1,124)	(870)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	354	400	708	800
Interest cost	474	466	948	932
Expected return on assets	(1,322)	(1,275)	(2,644)	(2,550)
Amortization of prior service credit	(350)	(349)	(700)	(698)
Amortization of net actuarial (gain) loss	(54)	6	(108)	12
Net periodic benefit credit, including amount capitalized	(898)	(752)	(1,796)	(1,504)
Less amount capitalized	53	44	84	83
Net periodic benefit cost (credit)	(951)	(796)	(1,880)	(1,587)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 773	$ 769	$ 1,500	$ 1,500