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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Computed tax at federal statutory rate        
Computed tax at federal statutory rate   $ 122,509 $ 114,179 $ 86,468
Federal statutory rate   35.00% 35.00% 35.00%
Increases (reductions) resulting from:        
State income taxes, net of federal income tax   $ 10,724 $ 9,027 $ 8,208
State income tax rate   3.10% 2.80% 3.30%
Federal renewable energy credit   $ (13,958) $ (13,544) $ (3,400)
Federal renewable energy credit rate   (4.00%) (4.20%) (1.40%)
Tax compliance and uncertain tax positions   $ (643) $ (3,028) $ (2,607)
Tax compliance and uncertain tax positions   (0.20%) (0.90%) (1.00%)
Domestic production deduction   $ (6,849) $ (6,251) $ (6,842)
Domestic production deduction, rate   (2.00%) (1.90%) (2.80%)
TCJA revaluation   $ (47,242) $ 0 $ 0
TCJA revaluation, rate   (13.50%) 0.00% 0.00%
TCJA revaluation related to accumulated other comprehensive loss balance $ (39,500) $ 7,735 $ 0 $ 0
TCJA revaluation related to accumulated other comprehensive loss balance, rate   2.20% 0.00% 0.00%
Other   $ (7,235) $ (7,251) $ (11,163)
Other rate   (2.00%) (2.30%) (4.50%)
Total income tax expense   $ 65,041 $ 93,132 $ 70,664
Total income tax expense rate   18.60% 28.50% 28.60%