Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	2,973	3,773
Expected return on assets	(4,761)	(5,120)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	1,863	1,355
Net periodic benefit cost (credit), including amount capitalized	75	8
Less amount capitalized	0	0
Net periodic benefit cost (credit)	75	8
Qualified plan | Overfunded Plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	352	344
Interest cost	591	780
Expected return on assets	(1,248)	(1,220)
Amortization of prior service credit	(297)	(349)
Amortization of net actuarial loss	2	97
Net periodic benefit cost (credit), including amount capitalized	(600)	(348)
Less amount capitalized	32	31
Net periodic benefit cost (credit)	(632)	(379)
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