Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 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	3,023	3,806	9,070	11,419
Expected return on assets	(4,987)	(4,559)	(14,962)	(13,677)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	1,793	1,387	5,379	4,160
Net periodic benefit cost (credit), including amount capitalized	(171)	634	(513)	1,902
Less amount capitalized	0	0	0	0
Net periodic benefit cost (credit)	(171)	634	(513)	1,902
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	383	286	1,149	857
Interest cost	609	746	1,827	2,239
Expected return on assets	(1,115)	(1,201)	(3,764)	(3,603)
Amortization of prior service credit	(349)	(289)	(1,048)	(866)
Amortization of net actuarial loss	71	27	215	82
Net periodic benefit cost (credit), including amount capitalized	(401)	(431)	(1,621)	(1,291)
Less amount capitalized	39	26	106	84
Net periodic benefit cost (credit)	(440)	(457)	(1,727)	(1,375)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,000	$ 1,100	$ 2,900	$ 3,300