Employee benefit plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	3,840	3,488	7,613	7,295
Expected return on assets	(3,998)	(5,379)	(9,118)	(10,377)
Amortization of prior service credit	0	0	0	0
Amortization of net actuarial (gain) loss	1,418	1,721	2,773	3,503
Net periodic benefit cost (credit), including amount capitalized	1,260	(170)	1,268	421
Less amount capitalized	0	0	0	0
Net periodic benefit cost (credit)	1,260	(170)	1,268	421
Qualified plan | Overfunded Plan [Member] | Other postretirement benefits
Defined benefit plan disclosure, net periodic benefit cost [Line Items]
Service cost	227	340	571	747
Interest cost	713	672	1,493	1,450
Expected return on assets	(1,182)	(1,266)	(2,402)	(2,433)
Amortization of prior service credit	(228)	(348)	(577)	(697)
Amortization of net actuarial (gain) loss	(42)	82	55	320
Net periodic benefit cost (credit), including amount capitalized	(512)	(520)	(860)	(613)
Less amount capitalized	27	43	58	83
Net periodic benefit cost (credit)	(539)	(563)	(918)	(696)
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	$ 2,200