Components of net periodic benefit cost (Details 3) - Underfunded Plan [Member] - Qualified Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 0
Interest cost	14,591	16,207	17,218
Expected return on assets	(20,753)	(20,528)	(20,924)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	7,005	6,355	6,215
Net periodic benefit cost (credit), including amount capitalized	843	2,034	2,509
Less amount capitalized	0	310	381
Net periodic benefit cost (credit)	843	1,724	2,128
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities):
Net (gain) loss	9,254	(5,827)	(3,789)
Amortization of actuarial loss	(7,005)	(6,355)	(6,215)
Amortization of prior service credit	0	0	0
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities)	2,249	(12,182)	(10,004)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities)	3,092	(10,458)	(7,876)
Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,494	1,508	1,647
Interest cost	2,899	3,265	3,688
Expected return on assets	(4,866)	(4,641)	(4,533)
Amortization of prior service credit	(1,394)	(1,371)	(1,371)
Recognized net actuarial loss	640	857	1,491
Net periodic benefit cost (credit), including amount capitalized	(1,227)	(382)	922
Less amount capitalized	153	(370)	(52)
Net periodic benefit cost (credit)	(1,380)	(12)	974
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive (income) loss or regulatory assets (liabilities):
Net (gain) loss	(2,467)	(3,190)	(3,523)
Amortization of actuarial loss	(640)	(857)	(1,491)
Amortization of prior service credit	1,394	1,371	1,371
Total recognized in accumulated other comprehensive (income) loss or regulatory assets (liabilities)	(1,713)	(2,676)	(3,643)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive (income) loss and regulatory assets (liabilities)	$ (3,093)	$ (2,688)	$ (2,669)