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SHARE-BASED PAYMENTS (Tables)
 12 Months Ended
Dec. 31, 2016
Share-based Payments [Line Items]  
Schedule of Share-based Compensation, Restricted Stock Units Activity
The following tables set forth activity and various statistics for restricted stock unit awards outstanding under the respective plans for the period indicated:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
231,258

 
$
32.59

Granted
 
42,935

 
$
58.30

Vested
 
(77,033
)
 
$
23.76

Forfeited
 
(2,260
)
 
$
38.04

Nonvested December 31, 2016
 
194,900

 
$
41.68

 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
58.30

 
$
41.40

 
$
33.19

Fair value of shares granted (thousands of dollars)
 
$
2,503

 
$
3,141

 
$
3,149
Schedule of Nonvested Performance-based Units Activity
The following tables set forth activity and various statistics related to our performance stock unit awards and the assumptions used by us in the valuations of the 2016, 2015 and 2014 grants at the grant date:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested December 31, 2015
 
439,250

 
$
27.35

Granted
 
74,395

 
$
64.06

Vested
 
(221,882
)
 
$
15.11

Forfeited
 
(2,952
)
 
$
41.44

Nonvested December 31, 2016
 
288,811

 
$
46.06

 
 
2016
 
2015
 
2014
 
Volatility (a)
 
18.20%
 
15.90%
 
18.40%
 
Dividend yield
 
2.40%
 
2.90%
 
3.37%
 
Risk-free interest rate
 
0.91%
 
1.10%
 
0.67%
 
(a) - Volatility based on historical volatility over three years using daily stock price observations of our peer utilities.
 
 
 
2016
 
2015
 
2014
Weighted-average grant date fair value (per share)
 
$
64.06

 
$
44.48

 
$
35.98

Fair value of shares granted (thousands of dollars)
 
$
4,766

 
$
4,486

 
$
4,462