Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 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	3,488	4,089	7,295	8,103
Expected return on assets	(5,379)	(5,234)	(10,377)	(10,263)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial loss	1,721	1,385	3,503	3,178
Net periodic benefit cost (credit), including amount capitalized	(170)	240	421	1,018
Less amount capitalized	0	73	0	180
Net periodic benefit cost (credit)	(170)	167	421	838
Qualified plan | Underfunded plan | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	340	306	747	753
Interest cost	672	825	1,450	1,633
Expected return on assets	(1,266)	(1,175)	(2,433)	(2,320)
Amortization of prior service credit	(348)	(343)	(697)	(686)
Amortization of net actuarial loss	82	100	320	436
Net periodic benefit cost (credit), including amount capitalized	(520)	(287)	(613)	(184)
Less amount capitalized	43	(114)	83	(153)
Net periodic benefit cost (credit)	(563)	(173)	(696)	(31)
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	$ 2,200	$ 2,300