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Income Taxes 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 factors impacting the difference between our effective tax rate and the federal statutory rate for the three months ended March 31 (dollars in thousands):
 
2019
 
2018
Federal income taxes at statutory rates
$
30,639

21.0
 %
 
$
13,790

21.0
 %
Increase (decrease) in tax resulting from:
 
 
 
 
 
Tax effect of regulatory treatment of utility plant differences
(2,080
)
(1.4
)
 
(1,018
)
(1.6
)
State income tax expense
1,659

1.1

 
964

1.5

Acquisition costs
(1,824
)
(1.2
)
 
46

0.1

Settlement of equity awards
612

0.4

 
(990
)
(1.5
)
Other
1,011

0.7

 
(2,082
)
(3.2
)
Total income tax expense
$
30,017

20.6
 %
 
$
10,710

16.3
 %