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SHARE-BASED PAYMENTS SHARE-BASED PAYMENTS (Tables)
 12 Months Ended
Dec. 31, 2017
Restricted Stock Units Activity  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Share-based Compensation, Activity
The following tables set forth activity and various statistics for our restricted stock unit awards:
 
 
Number of
Units
 
Weighted
Average Price
Nonvested December 31, 2016
 
881,647

 
$
31.25

Granted
 
281,167

 
$
45.11

Released to participants
 
(141,724
)
 
$
51.21

Forfeited
 
(19,285
)
 
$
32.07

Nonvested December 31, 2017
 
1,001,805

 
$
32.30

 
 
2017
 
2016
 
2015
Weighted-average grant date fair value (per share)
 
$
45.11

 
$
20.04

 
$
42.98

Fair value of units granted (thousands of dollars)
 
$
12,685

 
$
11,081

 
$
10,186

Fair value of units vested (thousands of dollars)
 
$
7,258

 
$
4,429

 
$
6,458
Performance-Unit Activity  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Schedule of Share-based Compensation, Activity
The following tables set forth activity and various statistics related to the performance unit awards and the assumptions used in the valuations at the respective grant dates:
 
 
Number of
Units
 
Weighted
Average Price
Nonvested December 31, 2016
 
1,005,751

 
$
38.81

Granted
 
311,047

 
$
56.65

Released to participants
 
(123,459
)
 
$
70.50

Forfeited
 
(57,206
)
 
$
42.29

Nonvested December 31, 2017
 
1,136,133

 
$
40.08

 
 
2017
 
2016
 
2015
Volatility (a)
 
40.59%
 
39.94%
 
26.70%
Dividend Yield
 
4.68%
 
11.32%
 
5.02%
Risk-free Interest Rate
 
1.49%
 
0.93%
 
1.00%
(a) - Volatility was based on historical volatility over three years using daily stock price observations.
 
 
2017
 
2016
 
2015
Weighted-average grant date fair value (per share)
 
$
56.65

 
$
25.54

 
$
50.30

Fair value of units granted (thousands of dollars)
 
$
17,621

 
$
15,229

 
$
13,370

Fair value of units vested (thousands of dollars)
 
$
8,704

 
$
—

 
$
13,736