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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Activity
The following table provides additional information concerning restricted stock activity, including performance-based and market-based shares, and stock options:
 
 
Three Months Ended March 31,
Restricted Stock
 
2018
 
2017
Weighted-average grant date fair value
 
$
29.03

 
$
22.12

Total fair value of restricted shares that vested (in thousands)
 
$
7,162

 
$
5,024

 
 
 
 
 
Stock Options
 
 
 
 
Weighted-average grant date fair value of options granted
 
$
—

 
$
—

Total fair value of options that vested (in thousands)
 
$
—

 
$
—

Total intrinsic value of options exercised (in thousands)
 
$
2,711

 
$
945

The following table summarizes the weighted-average assumptions used to determine the awards grant date fair value:
 
 
Three Months Ended March 31,
Restricted Shares and Performance Based Shares
 
2018
 
2017
Expected quarterly dividends per share
 
$
0.2650

 
$
0.2425

Risk-free interest rate
 
2.39
%
 
1.58
%
 
 
 
 
 
Market-Based Shares
 
 
 
 
Dividend yield
 
2.96
%
 
2.67
%
Expected volatility
 
19.12
%
 
20.80
%
Risk-free interest rate
 
2.36
%
 
1.54
%

The following table summarizes activity in restricted stock awards, including performance-based and market-based shares, and stock options, for the three months ended March 31, 2018:
 
Restricted Stock
 
Stock Options
 
Shares
 
Weighted-
Average
Grant Date Fair Value
 
Shares
 
Weighted-
Average
Exercise Price
Outstanding at December 31, 2017
189,045

 
$
31.11

 
193,441

 
$
9.98

Granted
204,654

 
$
29.03

 
—

 
$
—

Exercised
(201,162
)
 
$
27.68

 
(97,941
)
 
$
8.19

Forfeited
(3,562
)
 
$
30.58

 
—

 
$
—

Expired
—

 
$
—

 
—

 
$
—

Outstanding at March 31, 2018
188,975

 
$
32.52

 
95,500

 
$
11.81