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Income Taxes (Tables)
 3 Months Ended
Mar. 31, 2019
Income Tax Disclosure [Abstract]  
Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] 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,
 
2019
 
2018
Income Before Income Taxes
$
74,379

 
 
 
$
60,413

 
 
 
 
 
 
 
 
 
 
Income tax calculated at federal statutory rate
15,620

 
21.0
 %
 
12,687

 
21.0
 %
 
 
 
 
 
 
 
 
Permanent or flow-through adjustments:
 
 
 
 
 
 
 
State income, net of federal provisions
928

 
1.2

 
732

 
1.2

Flow-through repairs deductions
(7,935
)
 
(10.7
)
 
(6,586
)
 
(10.9
)
Production tax credits
(4,432
)
 
(6.0
)
 
(3,888
)
 
(6.4
)
Plant and depreciation of flow-through items
(1,523
)
 
(2.0
)
 
(916
)
 
(1.6
)
Amortization of excess deferred income tax
(1,376
)
 
(1.8
)
 
(384
)
 
(0.6
)
Share-based compensation
186

 
0.3

 
275

 
0.5

Other, net
105

 
0.1

 
(6
)
 
—

 
(14,047
)
 
(18.9
)
 
(10,773
)
 
(17.8
)
 
 
 
 
 
 
 
 
Income Tax Expense
$
1,573

 
2.1
 %
 
$
1,914

 
3.2
 %