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Income tax expense (benefit) statutory rate versus actual rate (Details 6) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Computed tax at federal statutory rate      
Computed tax at federal statutory rate $ 97,032 $ 97,997 $ 99,775
Federal statutory rate 21.00% 21.00% 21.00%
Increases (reductions) resulting from:      
State income taxes, net of federal income tax $ 19,126 $ 19,496 $ 17,845
State income tax, rate 4.10% 4.20% 3.80%
Federal renewable energy credit $ (15,343) $ (13,914) $ (16,009)
Federal renewable energy credit, rate (3.30%) (3.00%) (3.40%)
Tax compliance and uncertain tax positions $ 1,080 $ (477) $ (3,543)
Tax compliance and uncertain tax positions, rate 0.20% (0.10%) (0.70%)
Nonqualified benefit plans $ 2,827 $ (1,881) $ (2,443)
Effective Income Tax Rate Nonqualified benefit plans 0.60% (0.40%) (0.50%)
Excess deferred income tax amortization $ (9,008) $ (10,295) $ (12,517)
Excess deferred income tax amortization, rate (1.90%) (2.20%) (2.60%)
Other $ (931) $ (2,006) $ 1,482
Other, rate (0.20%) (0.40%) 0.20%
Income taxes $ 94,783 $ 88,920 $ 84,590
Total income tax expense, rate 20.50% 19.10% 17.80%