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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Share-based Payment Awards [Table Text Block]
Dollars in thousands
 
Shares(1)
 
Expense During Award Year(2)
 
Total Expense for Award
Estimated award:
 
 
 
 
 
 
2015-2017 grant(3)
 
18,300

 
$
(346
)
 
$
1,169

Actual award:
 
 
 
 
 
 
2014-2016 grant
 
31,388

 
168

 
1,685

2013-2015 grant
 
8,914

 
312

 
1,240
Schedule Of Performance Based Award [Table Text Block]
Dollars in thousands
 
Performance Share Awards Outstanding
 
2017
 
Cumulative Expense
Performance Period
 
Target
 
Maximum
 
Expense/(Reversal)
 
December 31, 2017
2015-17
 
29,967

 
59,934

 
$
(346
)
 
$
1,169

2016-18
 
24,826

 
49,652

 
337

 
815

2017-19
 
32,680

 
65,360

 
942

 
942

Total
 
87,473

 
174,946

 
$
933

 
 
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block]
 
 
Number of RSUs
 
Weighted -
Average
Price Per RSU
Nonvested, December 31, 2014
 
70,794

 
$
44.00

Granted
 
37,264

 
46.29

Vested
 
(19,003
)
 
44.81

Forfeited
 
(468
)
 
44.99

Nonvested, December 31, 2015
 
88,587

 
44.78

Granted
 
40,271

 
54.36

Vested
 
(29,488
)
 
45.56

Forfeited
 
(9,397
)
 
44.59

Nonvested, December 31, 2016
 
89,973

 
48.85

Granted
 
32,168

 
60.51

Vested
 
(35,341
)
 
47.07

Forfeited
 
(2,278
)
 
53.78

Nonvested, December 31, 2017
 
84,522

 
53.90
Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]
 
 
Option
Shares
 
Weighted -
Average
Price Per Share
 
Intrinsic
Value
(In millions)
Balance outstanding, December 31, 2014
 
416,088

 
$
43.40

 
$
2.7

Exercised
 
(62,900
)
 
39.96

 
0.5

Forfeited
 
(500
)
 
45.74

 
n/a

Balance outstanding, December 31, 2015
 
352,688

 
44.00

 
2.3

Exercised
 
(172,525
)
 
43.61

 
2.0

Forfeited
 
—

 
n/a

 
n/a

Balance outstanding, December 31, 2016
 
180,163

 
44.38

 
2.8

Exercised
 
(88,275
)
 
44.33

 
1.8

Forfeited
 
(200
)
 
41.15

 
n/a

Balance outstanding and exercisable, December 31, 2017
 
91,688

 
44.43

 
1.4
Schedule Of Stock Based Compensation Capitalized Under LTIP SOP and ESPP [Table Text Block]
In thousands
2017
2016
2015
Operations and maintenance expense, for stock-based compensation
$
2,354

$
2,370

$
2,673

Income tax benefit
(930
)
(924
)
(1,012
)
Net stock-based compensation effect on net income (loss)
$
1,424

$
1,446

$
1,661

Amounts capitalized for stock-based compensation
$
528

$
554

$
661