Components of net periodic benefit cost (Details 3) - Qualified Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Underfunded Plan | Pension Benefits
Components of net periodic benefit cost (credit):
Service cost	$ 0	$ 0	$ 0
Interest cost	12,093	15,225	14,591
Expected return on assets	(19,949)	(18,236)	(20,753)
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	7,172	5,548	7,005
Net periodic benefit cost (credit), including amount capitalized	(684)	2,537	843
Less amount capitalized	0	0	0
Net periodic benefit cost (credit)	(684)	2,537	843
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	934	(144)	991
Amortization of actuarial loss	(1,155)	(904)	(1,084)
Amortization of prior service (cost) credit	0	0	0
Total recognized in accumulated other comprehensive loss	(221)	(1,048)	(93)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	4,546	189	8,263
Amortization of actuarial gain (loss)	(6,017)	(4,644)	(5,921)
Amortization of prior service credit	0	0	0
Total recognized in regulatory assets or liabilities	(1,471)	(4,455)	2,342
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	(2,376)	(2,966)	3,092
Overfunded Plan | Other Postretirement Benefits
Components of net periodic benefit cost (credit):
Service cost	1,532	1,142	1,494
Interest cost	2,437	2,986	2,899
Expected return on assets	(5,019)	(4,804)	(4,866)
Amortization of prior service credit	(1,398)	(1,398)	(1,394)
Recognized net actuarial loss	287	353	640
Net periodic benefit cost (credit), including amount capitalized	(2,161)	(1,721)	(1,227)
Less amount capitalized	156	113	153
Net periodic benefit cost (credit)	(2,317)	(1,834)	(1,380)
Other changes in plan assets and benefit obligations recognized in accumulated comprehensive loss:
Net (gain) loss	(259)	(127)	(1,735)
Amortization of actuarial loss	(306)	(110)	(354)
Amortization of prior service (cost) credit	101	100	(220)
Total recognized in accumulated other comprehensive loss	(464)	(137)	(2,309)
Other changes in plan assets and benefit obligations recognized in regulatory assets or liabilities:
Net (gain) loss	(3,793)	(8,168)	(732)
Amortization of actuarial gain (loss)	19	(242)	(286)
Amortization of prior service credit	1,297	1,297	1,614
Total recognized in regulatory assets or liabilities	(2,477)	(7,113)	596
Total recognized in net periodic benefit cost (credit), accumulated other comprehensive loss and regulatory assets or liabilities	$ (5,258)	$ (9,084)	$ (3,093)