Employee benefit plans (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	2,455,000	2,973,000
Expected return on assets	(4,894,000)	(4,761,000)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	2,004,000	1,863,000
Net periodic benefit cost (credit), including amount capitalized	(435,000)	75,000
Less amount capitalized	0	0
Net periodic benefit cost (credit)	(435,000)	75,000
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	400,000	352,000
Interest cost	466,000	591,000
Expected return on assets	(1,275,000)	(1,248,000)
Amortization of prior service credit	(349,000)	(297,000)
Amortization of net actuarial loss	6,000	2,000
Net periodic benefit cost (credit), including amount capitalized	(752,000)	(600,000)
Less amount capitalized	39,000	32,000
Net periodic benefit cost (credit)	(791,000)	(632,000)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 769,000	$ 1,000,000.0