Employee benefit plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Interest cost	$ 2,630,000	$ 2,455,000	$ 7,892,000	$ 7,365,000
Expected return on assets	(4,864,000)	(4,894,000)	(14,592,000)	(14,682,000)
Amortization of net actuarial (gain) loss	1,670,000	2,004,000	5,012,000	6,012,000
Net periodic benefit cost (credit)	(564,000)	(435,000)	(1,688,000)	(1,305,000)
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	354,000	400,000	1,062,000	1,200,000
Interest cost	474,000	466,000	1,422,000	1,398,000
Expected return on assets	(1,322,000)	(1,275,000)	(3,966,000)	(3,825,000)
Amortization of prior service credit	(350,000)	(349,000)	(1,050,000)	(1,047,000)
Amortization of net actuarial (gain) loss	(55,000)	6,000	(163,000)	18,000
Net periodic benefit credit, including amount capitalized	(899,000)	(752,000)	(2,695,000)	(2,256,000)
Less amount capitalized	48,000	46,000	132,000	129,000
Net periodic benefit cost (credit)	(947,000)	(798,000)	(2,827,000)	(2,385,000)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 773,000	$ 769,000	$ 2,300,000	$ 2,300,000