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SHARE-BASED COMPENSATION (Tables)
 12 Months Ended
Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Schedule of Share-based Compensation Cost Allocation
We recorded share-based compensation in 2015, 2014 and 2013 as follows:
(In thousands)
2015
 
2014
 
2013
Operation and maintenance expenses
$
1,672

 
$
1,444

 
$
1,617

General and administrative expenses
10,546

 
8,549

 
9,318

Amounts capitalized to property, plant and equipment
5,391

 
4,659

 
4,731

Total share-based compensation
$
17,609

 
$
14,652

 
$
15,666

Total tax benefit recognized in the consolidated statement of operations
$
5,087

 
$
4,182

 
$
4,557
Schedule of Stock Options Activity
Stock option activity for 2015 was as follows:
 
 
 
Weighted
 
Number of
 
Average
 
Options
 
Exercise Price
Outstanding at January 1, 2015 (3,198,528 exercisable with a weighted average exercise price of $15.98)
4,603,292

 
$
20.71

Granted
473,200

 
35.91

Exercised
(1,203,376
)
 
9.44

Forfeited
(55,916
)
 
34.65

Outstanding at December 31, 2015 (2,718,865 exercisable with a weighted average exercise price of $22.38)
3,817,200

 
$
25.94
Stock Options  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Schedule of Stock Options Valuation Assumptions
The following assumptions were used in determining the weighted average fair value per option:
 
2015
 
2014
 
2013
 
Option Grants
 
Option Grants
 
Option Grants
Weighted average grant date fair value per option
$
6.05

 
$
8.92

 
$
7.06

Weighted average expected volatility (a)
18.6
%
 
27.2
%
 
29.3
%
Weighted average risk-free interest rate
1.8
%
 
1.8
%
 
1.1
%
Weighted average expected term (b)
6 years

 
6 years

 
6 years

Weighted average expected dividend yield
1.59
%
 
1.55
%
 
1.72
%
Estimated fair value of underlying shares
$
35.91

 
$
36.73

 
$
29.31

____________________________
(a)
We estimated volatility using the historical volatility of our stock.
(b)
The expected term represents the period of time that options granted are expected to be outstanding. We have utilized the simplified method permitted under share-based award accounting standards in determining the expected term for all option grants as we do not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited number of awards of equity shares that have reached expiration.
Restricted Stock  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Schedule of Restricted Stock Activity
Restricted stock award activity for 2015 was as follows:
 
Number of
 
Weighted
 
Restricted
 
Average
 
Stock
 
Grant Date
 
Awards
 
Fair Value
Unvested restricted stock awards at January 1, 2015
1,223,819

 
$
28.37

Granted
259,039

 
36.30

Vested
(400,239
)
 
23.63

Forfeited
(58,209
)
 
26.65

Unvested restricted stock awards at December 31, 2015
1,024,410

 
$
32.10
Performance Shares  
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]  
Summary of Unvested Performance Share Activity
 
Number of
 
Weighted
 
Performance
 
Average
 
Share
 
Grant Date
 
Awards
 
Fair Value
Unvested performance share awards at January 1, 2015
—

 
$
—

Granted
287,464

 
32.55

Vested
—

 
—

Forfeited
(7,754
)
 
32.55

Unvested performance share awards at December 31, 2015
279,710

 
$
32.55
Schedule of Performance Shares Valuation Assumptions
The following assumptions were used in determining the weighted average fair value per performance share with the TSR condition:
 
2015
 
Performance
 
Share Grants
Weighted average grant date fair value per performance share
$
29.19

Weighted average expected volatility (a)
17.7
%
Weighted average risk-free interest rate
0.87
%
Estimated fair value of underlying shares
$
35.91