Employee benefit plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	3,074,000	3,840,000	6,047,000	7,613,000
Expected return on assets	(5,214,000)	(3,998,000)	(9,975,000)	(9,118,000)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial (gain) loss	1,723,000	1,418,000	3,586,000	2,773,000
Net periodic benefit cost (credit), including amount capitalized	(417,000)	1,260,000	(342,000)	1,268,000
Less amount capitalized	0	0	0	0
Net periodic benefit cost (credit)	(417,000)	1,260,000	(342,000)	1,268,000
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	414,000	227,000	766,000	571,000
Interest cost	627,000	713,000	1,218,000	1,493,000
Expected return on assets	(1,401,000)	(1,182,000)	(2,649,000)	(2,402,000)
Amortization of prior service credit	(402,000)	(228,000)	(699,000)	(577,000)
Amortization of net actuarial (gain) loss	142,000	(42,000)	144,000	55,000
Net periodic benefit cost (credit), including amount capitalized	(620,000)	(512,000)	(1,220,000)	(860,000)
Less amount capitalized	35,000	27,000	67,000	58,000
Net periodic benefit cost (credit)	(655,000)	(539,000)	(1,287,000)	(918,000)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 900,000	$ 1,100,000	$ 1,900,000	$ 2,200,000