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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock-Based Compensation Expense
Stock-based compensation expense is summarized as follows: 
 
Years Ended December 31,
 
2017
 
2016
 
2015
($ in thousands)
 
 
 
 
 
Stock-based compensation expense
$
20,288

 
$
11,948

 
$
11,863
Summary of Restricted Stock Units Activity
RSU activity for the year ended December 31, 2017 is summarized as follows: 
 
Number
of shares
 
Weighted Average
Grant Date
Fair Value
Outstanding at December 31, 2016
302,824

 
$
111.56

Granted
290,630

 
$
108.32

Forfeited
(30,450
)
 
$
120.08

Settled
(79,983
)
 
$
141.24

Outstanding at December 31, 2017
483,021

 
$
104.16
Summary of Stock Option Activity
Stock option activity for the year ended December 31, 2017 is summarized as follows: 
 
Number
of shares
 
Weighted
Average
Exercise Price
Outstanding at December 31, 2016
137,157

 
$
17.77

Granted
—

 
$
—

Exercised
(27,349
)
 
$
23.12

Forfeited
—

 
$
—

Outstanding at December 31, 2017
109,808

 
$
16.44

Vested and exercisable at December 31, 2017
109,808

 
$
16.44