Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 99,775	$ 83,674	$ 66,543
Federal statutory rate	21.00%	21.00%	21.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 17,845	$ 14,029	$ 12,190
State income tax, rate	3.80%	3.50%	3.80%
Federal renewable energy credit	$ (16,009)	$ (15,843)	$ (11,759)
Federal renewable energy credit, rate	(3.40%)	(4.00%)	(3.70%)
Tax compliance and uncertain tax positions	$ (3,543)	$ (2,739)	$ (2,725)
Tax compliance and uncertain tax positions, rate	(0.70%)	(0.70%)	(0.90%)
Excess deferred income tax amortization	$ (12,517)	$ (11,904)	$ (9,319)
Excess deferred income tax amortization, rate	(2.60%)	(3.00%)	(2.90%)
TCJA revaluation	$ 0	$ 0	$ (5,947)
TCJA revaluation, rate	0.00%	0.00%	(1.90%)
TCJA revaluation related to accumulated other comprehensive loss balance	$ 0	$ 0	$ (42)
TCJA revaluation related to accumulated other comprehensive loss balance, rate	0.00%	0.00%	0.00%
Other	$ (961)	$ (3,938)	$ (1,456)
Other, rate	(0.30%)	(0.90%)	(0.40%)
Income taxes	$ 84,590	$ 63,279	$ 47,485
Total income tax expense, rate	17.80%	15.90%	15.00%

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

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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