Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	3,648	4,052	10,943	12,155
Expected return on assets	(5,188)	(5,132)	(15,565)	(15,395)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	1,751	1,589	5,254	4,767
Net periodic benefit cost (credit), including amount capitalized	211	509	632	1,527
Less amount capitalized	0	65	0	245
Net periodic benefit cost (credit)	211	444	632	1,282
Qualified plan | Underfunded plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	374	377	1,121	1,130
Interest cost	725	816	2,175	2,449
Expected return on assets	(1,217)	(1,160)	(3,650)	(3,480)
Amortization of prior service credit	(349)	(343)	(1,046)	(1,029)
Amortization of net actuarial loss	160	213	480	649
Net periodic benefit cost (credit), including amount capitalized	(307)	(97)	(920)	(281)
Less amount capitalized	46	(95)	128	(248)
Net periodic benefit cost (credit)	(353)	(2)	(1,048)	(33)
Nonqualified plan | Unfunded plan | Supplemental employee retirement plans
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Net periodic benefit cost (credit)	$ 1,100	$ 1,200	$ 3,300	$ 3,500