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Income Taxes Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2018
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 rates (21 percent in 2018 and 35 percent in 2017) for three and nine months ended September 30 (dollars in thousands):
 
Three months ended September 30,
 
Nine months ended September 30,
 
2018
 
2017
 
2018
 
2017
Federal income taxes at statutory rates
$
2,452

21.0
 %
 
$
3,364

35.0
 %
 
$
22,721

21.0
 %
 
$
48,953

35.0
 %
Increase (decrease) in tax resulting from:
 
 
 
 
 
 
 
 
 
 
 
Tax effect of regulatory treatment of utility plant differences
(1,521
)
(13.0
)
 
742

7.7

 
(4,519
)
(4.2
)
 
2,244

1.6

State income tax expense
(319
)
(2.7
)
 
(301
)
(3.1
)
 
694

0.6

 
1,136

0.8

Acquisition costs
122

1.0

 
1,997

20.8

 
241

0.2

 
2,317

1.7

Settlement of equity awards
—

—

 
—

—

 
(990
)
(0.9
)
 
(1,439
)
(1.0
)
Other
814

7.0

 
(649
)
(6.8
)
 
(680
)
(0.6
)
 
(1,663
)
(1.2
)
Total income tax expense
$
1,548

13.3
 %
 
$
5,153

53.6
 %
 
$
17,467

16.1
 %
 
$
51,548

36.9
 %