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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2018
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,
($ in thousands)
2018
 
2017
 
2016
Stock-based compensation expense
$
23,116

 
$
20,288

 
$
11,948
Summary of Restricted Stock Units Activity
RSU activity for the year ended December 31, 2018 is summarized as follows: 
 
Number
of shares
 
Weighted Average
Grant Date
Fair Value
Outstanding at December 31, 2017
483,021

 
$
104.16

Granted
198,180

 
$
131.16

Forfeited
(19,814
)
 
$
134.65

Settled
(109,149
)
 
$
114.28

Outstanding at December 31, 2018
552,238

 
$
111.49
Summary of Stock Option Activity
Stock option activity for the year ended December 31, 2018 is summarized as follows: 
 
Number
of shares
 
Weighted
Average
Exercise Price
Outstanding at December 31, 2017
109,808

 
$
16.44

Exercised
(33,057
)
 
$
24.74

Outstanding at December 31, 2018
76,751

 
$
12.86

Vested and exercisable at December 31, 2018
76,751

 
$
12.86