Components of net periodic benefit cost (Details 3) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 86
Interest cost	16,207	17,218	17,141
Expected return on assets	(20,528)	(20,924)	(22,254)
Amortization of prior service cost (credit)	0	0	36
Recognized net actuarial loss	6,355	6,215	7,016
Curtailment loss	0	0	258
Net periodic benefit cost (credit), including amount capitalized	2,034	2,509	2,283
Less amount capitalized	310	381	316
Net periodic benefit cost (credit)	1,724	2,128	1,967
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities):
Net (gain) loss	(5,827)	(3,789)	8,257
Amortization of actuarial loss	(6,355)	(6,215)	(7,016)
Amortization of prior service (cost) credit	0	0	(294)
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities)	(12,182)	(10,004)	947
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities)	(10,458)	(7,876)	2,914
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,508	1,647	1,816
Interest cost	3,265	3,688	3,607
Expected return on assets	(4,641)	(4,533)	(4,795)
Amortization of prior service cost (credit)	(1,371)	(1,371)	(1,371)
Recognized net actuarial loss	857	1,491	1,960
Curtailment loss	0	0	0
Net periodic benefit cost (credit), including amount capitalized	(382)	922	1,217
Less amount capitalized	(370)	(52)	120
Net periodic benefit cost (credit)	(12)	974	1,097
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities):
Net (gain) loss	(3,190)	(3,523)	(1,336)
Amortization of actuarial loss	(857)	(1,491)	(1,960)
Amortization of prior service (cost) credit	1,371	1,371	1,371
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities)	(2,676)	(3,643)	(1,925)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities)	$ (2,688)	$ (2,669)	$ (828)