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Income Taxes
 3 Months Ended
Mar. 31, 2018
Income Tax Disclosure [Abstract]  
Income Tax Disclosure [Text Block]
Income Taxes
 
The primary impact of the Tax Cuts and Jobs Act is a reduction of the federal corporate income tax rate from 35% to 21% effective January 1, 2018. We compute income tax expense for each quarter based on the estimated annual effective tax rate for the year, adjusted for certain discrete items. Our effective tax rate typically differs from the federal statutory tax rate due to the regulatory impact of flowing through the federal and state tax benefit of repairs deductions, state tax benefit of accelerated tax depreciation deductions (including bonus depreciation when applicable) and production tax credits. The regulatory accounting treatment of these deductions requires immediate income recognition for temporary tax differences of this type, which is referred to as the flow-through method. When the flow-through method of accounting for temporary differences is reflected in regulated revenues, we record deferred income taxes and establish related regulatory assets and liabilities. We revalued our deferred tax assets and liabilities as of December 31, 2017, which reflected our estimate of the impact of the Tax Cuts and Jobs Act. We will continue to evaluate subsequent regulations, clarifications and interpretations with the assumptions made, which could materially change our estimate.

The following table summarizes the significant differences in income tax expense based on the differences between our effective tax rate and the federal statutory rate (in thousands):
 
Three Months Ended March 31,
 
2018
 
2017
Income Before Income Taxes
$
60,413

 
 
 
$
63,244

 
 
 
 
 
 
 
 
 
 
Income tax calculated at federal statutory rate
12,687

 
21.0
 %
 
22,135

 
35.0
 %
 
 
 
 
 
 
 
 
Permanent or flow through adjustments:
 
 
 
 
 
 
 
State income, net of federal provisions
732

 
1.2

 
(834
)
 
(1.3
)
Flow-through repairs deductions
(6,586
)
 
(10.9
)
 
(8,797
)
 
(13.9
)
Production tax credits
(3,888
)
 
(6.4
)
 
(3,831
)
 
(6.1
)
Plant and depreciation of flow through items
(916
)
 
(1.6
)
 
(1,440
)
 
(2.3
)
Share-based compensation
275

 
0.5

 
(399
)
 
(0.6
)
Other, net
(390
)
 
(0.6
)
 
(157
)
 
(0.2
)
 
(10,773
)
 
(17.8
)
 
(15,458
)
 
(24.4
)
 
 
 
 
 
 
 
 
Income Tax Expense
$
1,914

 
3.2
 %
 
$
6,677

 
10.6
 %

 
 
 
 
 
 
 
 
Uncertain Tax Positions

We recognize tax positions that meet the more-likely-than-not threshold as the largest amount of tax benefit that is greater than 50 percent likely of being realized upon ultimate settlement with a taxing authority that has full knowledge of all relevant information. We have unrecognized tax benefits of approximately $56.5 million as of March 31, 2018, including approximately $47.7 million that, if recognized, would impact our effective tax rate. We do not anticipate that total unrecognized tax benefits will significantly change due to the settlement of audits or the expiration of statutes of limitation within the next twelve months.

Our policy is to recognize interest and penalties related to uncertain tax positions in income tax expense. During the three months ended March 31, 2018 and 2017, we recognized $0.3 million and $0.1 million, respectively, of expense for interest and penalties in the Condensed Consolidated Statements of Income. As of March 31, 2018 and December 31, 2017, we had $1.7 million and $1.5 million, respectively, of interest accrued in the Condensed Consolidated Balance Sheets.

Our federal tax returns from 2000 forward remain subject to examination by the IRS.