Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	3,773	3,807
Expected return on assets	(5,120)	(4,998)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	1,355	1,782
Net periodic benefit cost (credit), including amount capitalized	8	591
Less amount capitalized	0	0
Net periodic benefit cost (credit)	8	591
Qualified plan | Overfunded Plan [Member] | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	344	407
Interest cost	780	778
Expected return on assets	(1,220)	(1,167)
Amortization of prior service credit	(252)	(349)
Amortization of net actuarial loss	0	238
Net periodic benefit cost (credit), including amount capitalized	(348)	(93)
Less amount capitalized	31	40
Net periodic benefit cost (credit)	(379)	(133)
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