Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 2,631	$ 2,455
Expected return on assets	(4,864)	(4,894)
Amortization of net actuarial (gain) loss	1,671	2,004
Net periodic benefit cost (credit)	(562)	(435)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	354	400
Interest cost	474	466
Expected return on assets	(1,322)	(1,275)
Amortization of prior service credit	(350)	(349)
Amortization of net actuarial (gain) loss	(54)	6
Net periodic benefit credit, including amount capitalized	(898)	(752)
Less amount capitalized	31	39
Net periodic benefit cost (credit)	(929)	(791)
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