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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, 2018	Dec. 31, 2017	Dec. 31, 2016
Computed tax at federal statutory rate
Computed tax at federal statutory rate		$ 66,543	$ 122,509	$ 114,179
Federal statutory rate		21.00%	35.00%	35.00%
Increases (reductions) resulting from:
State income taxes, net of federal income tax		$ 12,190	$ 10,724	$ 9,027
State income tax, rate		3.80%	3.10%	2.80%
Federal renewable energy credit		$ (11,759)	$ (13,958)	$ (13,544)
Federal renewable energy credit, rate		(3.70%)	(4.00%)	(4.20%)
Tax compliance and uncertain tax positions		$ (2,725)	$ (643)	$ (3,028)
Tax compliance and uncertain tax positions, rate		(0.90%)	(0.20%)	(0.90%)
Domestic production deduction		$ 0	$ (6,849)	$ (6,251)
Domestic production deduction, rate		0.00%	(2.00%)	(1.90%)
Excess deferred income tax amortization		$ (9,319)	$ (397)	$ (828)
Excess deferred income tax amortization, rate		(2.90%)	0.00%	(0.20%)
TCJA revaluation		$ (5,947)	$ (47,242)	$ 0
TCJA revaluation, rate		(1.90%)	(13.50%)	0.00%
TCJA revaluation related to accumulated other comprehensive loss balance	$ (39,500)	$ (42)	$ 7,735	$ 0
TCJA revaluation related to accumulated other comprehensive loss balance, rate		0.00%	2.20%	0.00%
Other		$ (1,456)	$ (6,838)	$ (6,423)
Other, rate		(0.40%)	(2.00%)	(2.10%)
Total income tax expense		$ 47,485	$ 65,041	$ 93,132
Total income tax expense, rate		15.00%	18.60%	28.50%

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