Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Qualified plan | Underfunded plan | Pension benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	$ 0	$ 0
Interest cost	3,807	4,014
Expected return on assets	(4,998)	(5,029)
Amortization of prior service credit	0	0
Amortization of net actuarial loss	1,782	1,793
Net periodic benefit cost (credit), including amount capitalized	591	778
Less amount capitalized	0	107
Net periodic benefit cost (credit)	591	671
Qualified plan | Underfunded plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	407	447
Interest cost	778	808
Expected return on assets	(1,167)	(1,145)
Amortization of prior service credit	(349)	(343)
Amortization of net actuarial loss	238	336
Net periodic benefit cost (credit), including amount capitalized	(93)	103
Less amount capitalized	40	(39)
Net periodic benefit cost (credit)	(133)	142
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