Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Computed tax at federal statutory rate
Computed tax at federal statutory rate	$ 97,997	$ 99,775	$ 83,674
Federal statutory rate	21.00%	21.00%	21.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax	$ 19,496	$ 17,845	$ 14,029
State income tax, rate	4.20%	3.80%	3.50%
Federal renewable energy credit	$ (13,914)	$ (16,009)	$ (15,843)
Federal renewable energy credit, rate	(3.00%)	(3.40%)	(4.00%)
Tax compliance and uncertain tax positions	$ (477)	$ (3,543)	$ (2,739)
Tax compliance and uncertain tax positions, rate	(0.10%)	(0.70%)	(0.70%)
Excess deferred income tax amortization	$ (10,295)	$ (12,517)	$ (11,904)
Excess deferred income tax amortization, rate	(2.20%)	(2.60%)	(3.00%)
Other	$ (3,887)	$ (961)	$ (3,938)
Other, rate	(0.80%)	(0.30%)	(0.90%)
Income taxes	$ 88,920	$ 84,590	$ 63,279
Total income tax expense, rate	19.10%	17.80%	15.90%