Income tax expense (benefit) statutory rate versus actual rate (Details 7) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Computed tax at federal statutory rate
Computed tax at federal statutory rate		$ 83,674	$ 66,543	$ 122,509
Federal statutory rate		21.00%	21.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax		$ 14,029	$ 12,190	$ 10,724
State income tax, rate		3.50%	3.80%	3.10%
Federal renewable energy credit		$ (15,843)	$ (11,759)	$ (13,958)
Federal renewable energy credit, rate		(4.00%)	(3.70%)	(4.00%)
Tax compliance and uncertain tax positions		$ (2,739)	$ (2,725)	$ (643)
Tax compliance and uncertain tax positions, rate		(0.70%)	(0.90%)	(0.20%)
Domestic production deduction		$ 0	$ 0	$ (6,849)
Domestic production deduction, rate		0.00%	0.00%	(2.00%)
Excess deferred income tax amortization		$ (11,904)	$ (9,319)	$ (397)
Excess deferred income tax amortization, rate		(3.00%)	(2.90%)	0.00%
TCJA revaluation		$ 0	$ (5,947)	$ (47,242)
TCJA revaluation, rate		0.00%	(1.90%)	(13.50%)
TCJA revaluation related to accumulated other comprehensive loss balance	$ 39,500	$ 0	$ (42)	$ 7,735
TCJA revaluation related to accumulated other comprehensive loss balance, rate		0.00%	0.00%	2.20%
Other		$ (3,938)	$ (1,456)	$ (6,838)
Other, rate		(0.90%)	(0.40%)	(2.00%)
Income taxes		$ 63,279	$ 47,485	$ 65,041
Total income tax expense, rate		15.90%	15.00%	18.60%

Effective Income Tax Rate Regulatory Liability, Excess Deferred Income Tax Benefit Amortization

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