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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Expense Related to Share-Based Payments
The table below summarizes the expense recognized related to share-based payments recognized for the three and six month periods ended June 30 (in thousands): 
 
 
Three Months Ended
June 30,
 
Six Months Ended
June 30,
 
 
2018
 
2017
 
2018
 
2017
Stock options
 
$
11,352

 
$
13,949

 
$
22,051

 
$
26,038

Restricted stock
 
7,750

 
7,201

 
11,454

 
18,205

Stock-based compensation
 
$
19,102


$
21,150


$
33,505


$
44,243
Summary of Total Unrecognized Compensation Cost Related to Stock-Based Compensation
The following table summarizes the Company’s total unrecognized compensation cost related to stock-based compensation as of June 30, 2018 (cost in thousands):
 
 
Unrecognized
Compensation
Cost
 
Weighted Average
Period of Expense
Recognition
(in Years)
Stock options
 
$
77,704

 
1.60
Restricted stock
 
22,109

 
0.71
Total
 
$
99,813

 
 
Summary of Changes in Number of Shares of Common Stock Under Option
The following summarizes the changes in the number of shares of common stock under option for the six month period ended June 30, 2018 (shares and aggregate intrinsic value in thousands):
 
 
Shares
 
Weighted
Average
Exercise
Price
 
Options
Exercisable
at End of
Period
 
Weighted
Average
Exercise
Price of
Exercisable
Options
 
Weighted
Average Fair
Value of
Options
Granted 
During the Period
 
Aggregate
Intrinsic
Value
Outstanding at December 31, 2017
 
8,031

 
$
109.78

 
4,029

 
$
75.80

 
 
 
$
663,815

Granted
 
349

 
202.61

 
 
 
 
 
$
202.68

 
 
Exercised
 
(420
)
 
73.08

 
 
 
 
 
 
 
57,825

Forfeited
 
(35
)
 
138.91

 
 
 
 
 
 
 
 
Outstanding at June 30, 2018
 
7,925

 
$
115.68

 
4,702

 
$
88.02

 
 
 
$
752,617

Expected to vest as of June 30, 2018
 
7,925

 
$
115.68

 
 
 
 
 
 
 
 
Schedule of Weighted-Average Assumptions
The fair value of stock option awards granted was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions for grants or modifications during the six month periods ended June 30, 2018 and 2017:
 
 
June 30,
 
 
2018
 
2017
Risk-free interest rate
 
2.55
%
 
1.65
%
Dividend yield
 
—

 
—

Expected volatility
 
26.96
%
 
28.00
%
Expected life (in years)
 
3.9

 
3.4
Summary of Changes in Number of Shares of Restricted Stock and Restricted Stock Units
The following table summarizes the changes in the number of shares of restricted stock and restricted stock units for the six months ended June 30, 2018 (shares in thousands): 
 
 
Shares
 
Weighted
Average
Grant Date
Fair Value
Outstanding at December 31, 2017
 
365

 
$
155.58

Granted
 
96

 
205.33

Vested
 
(48
)
 
158.79

Canceled or forfeited
 
(29
)
 
156.17

Outstanding at June 30, 2018
 
384

 
$
158.01