Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Underfunded Plan | Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 0
Interest cost	15,225	14,591	16,207
Expected return on assets	(18,236)	(20,753)	(20,528)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	5,548	7,005	6,355
Net periodic benefit cost (credit), including amount capitalized	2,537	843	2,034
Less amount capitalized	0	0	310
Net periodic benefit cost (credit)	2,537	843	1,724
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(144)	991	(1,091)
Amortization of actuarial loss	(904)	(1,084)	(1,040)
Amortization of prior service (cost) credit	0	0	0
Total recognized in accumulated other comprehensive loss	(1,048)	(93)	(2,131)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	189	8,263	(4,736)
Amortization of actuarial loss	(4,644)	(5,921)	(5,315)
Amortization of prior service credit	0	0	0
Total recognized in regulatory assets or liabilities	(4,455)	2,342	(10,051)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	(2,966)	3,092	(10,458)
Overfunded Plan | Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,142	1,494	1,508
Interest cost	2,986	2,899	3,265
Expected return on assets	(4,804)	(4,866)	(4,641)
Amortization of prior service credit	(1,398)	(1,394)	(1,371)
Recognized net actuarial loss	353	640	857
Net periodic benefit cost (credit), including amount capitalized	(1,721)	(1,227)	(382)
Less amount capitalized	113	153	(370)
Net periodic benefit cost (credit)	(1,834)	(1,380)	(12)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(127)	(1,735)	1,742
Amortization of actuarial loss	(110)	(354)	(289)
Amortization of prior service (cost) credit	100	(220)	161
Total recognized in accumulated other comprehensive loss	(137)	(2,309)	1,614
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	(8,168)	(732)	(4,932)
Amortization of actuarial loss	(242)	(286)	(568)
Amortization of prior service credit	1,297	1,614	1,210
Total recognized in regulatory assets or liabilities	(7,113)	596	(4,290)
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	$ (9,084)	$ (3,093)	$ (2,688)