Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	3,806	3,648	11,419	10,943
Expected return on assets	(4,559)	(5,188)	(13,677)	(15,565)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	1,387	1,751	4,160	5,254
Net periodic benefit cost (credit), including amount capitalized	634	211	1,902	632
Less amount capitalized	0	0	0	0
Net periodic benefit cost (credit)	634	211	1,902	632
Qualified plan | Overfunded Plan [Member] | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	286	374	857	1,121
Interest cost	746	725	2,239	2,175
Expected return on assets	(1,201)	(1,217)	(3,603)	(3,650)
Amortization of prior service credit	(289)	(349)	(866)	(1,046)
Amortization of net actuarial loss	27	160	82	480
Net periodic benefit cost (credit), including amount capitalized	(431)	(307)	(1,291)	(920)
Less amount capitalized	26	46	84	128
Net periodic benefit cost (credit)	(457)	(353)	(1,375)	(1,048)
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	$ 1,100	$ 3,300	$ 3,300