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SHARE-BASED PAYMENTS (Tables)
 12 Months Ended
Dec. 31, 2018
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 at December 31, 2017
 
140,665

 
$
51.97

Granted
 
37,893

 
$
68.17

Vested
 
(66,543
)
 
$
41.92

Forfeited
 
(2,509
)
 
$
62.44

Nonvested at December 31, 2018
 
109,506

 
$
63.45

 
 
2018
 
2017
 
2016
Weighted-average grant date fair value (per share)
 
$
68.17

 
$
63.97

 
$
58.30

Fair value of shares granted (thousands of dollars)
 
$
2,583

 
$
2,420

 
$
2,503
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 2018, 2017 and 2016 grants at the grant date:
 
 
Number of
Units
 
Weighted-
Average Price
Nonvested at December 31, 2017
 
237,324

 
$
57.78

Granted
 
79,447

 
$
74.04

Vested
 
(93,976
)
 
$
44.48

Forfeited
 
(3,464
)
 
$
67.97

Nonvested at December 31, 2018
 
219,331

 
$
69.21

 
 
2018
 
2017
 
2016
 
Volatility (a)
 
18.80%
 
20.70%
 
18.20%
 
Dividend yield
 
2.70%
 
2.63%
 
2.40%
 
Risk-free interest rate
 
2.38%
 
1.48%
 
0.91%
 
(a) - Volatility based on historical volatility over three years using daily stock price observations of our peer utilities.
 
 
 
2018
 
2017
 
2016
Weighted-average grant date fair value (per share)
 
$
74.04

 
$
68.94

 
$
64.06

Fair value of shares granted (thousands of dollars)
 
$
5,882

 
$
5,110

 
$
4,766