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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Schedule of Share-based Compensation, Restricted Stock Units Award Activity
RSU activity for the nine months ended September 30, 2017 is as follows:

 
Number of
 
Weighted-Average
Grant-Date
(in thousands, except weighted-average fair value prices)
Shares
 
Fair Value
Unvested RSUs outstanding at December 31, 2016
1,555

 
$
34.14

RSUs granted
672

 
$
39.99

RSUs vested
(858
)
 
$
34.29

RSUs forfeited
(61
)
 
$
35.87

Unvested RSUs outstanding at September 30, 2017
1,308

 
$
37.22
Schedule of Share-based Payment Award, Performance-Based Units, Valuation Assumptions
The fair values of the 2017 and 2016 awards were estimated using Monte-Carlo simulation with the following weighted-average assumptions:

 
For the Nine Months Ended September 30,
 
2017
 
2016
 
 
 
 
Expected dividend yield
—
%
 
—
 %
Risk-free interest rate (1)
1.47
%
 
0.99
 %
Expected volatility (2)
27.83
%
 
25.12
 %
Average total stockholder return (2)
1.46
%
 
(1.23
)%

(1)
The risk-free interest rate for the periods within the contractual term of the PBRSUs is based on the U.S. Treasury yield curve in effect at the time of the grant.
(2)
The expected volatility and average total stockholder return are measures of the amount by which a stock price has fluctuated or is expected to fluctuate based primarily on our and our peers' historical data.
Schedule of Other Share-based Compensation, Activity
PBRSU activity for the nine months ended September 30, 2017 is as follows:

 
Number of
 
Weighted-Average
Grant-Date
(in thousands, except weighted-average fair value prices)
Shares
 
Fair Value
Unvested PBRSUs outstanding at December 31, 2016
738

 
$
34.13

PBRSUs granted
288

 
$
39.79

PBRSUs vested
(227
)
 
$
31.90

PBRSUs forfeited
(159
)
 
$
36.48

Unvested PBRSUs outstanding at September 30, 2017
640

 
$
36.91
Schedule of Share-based Compensation, Stock Options, Activity
Option activity for the nine months ended September 30, 2017 is as follows:

(in thousands, except weighted-average price)
Number of
Shares
 
Weighted-Average
Exercise Price
 
Weighted-Average
Remaining
Contractual Term
 
Aggregate
Intrinsic
Value
Options outstanding at December 31, 2016
1,504

 
$
21.22

 
 
 
 
Options exercised
(215
)
 
$
24.69

 
 
 
 
Options vested, exercisable, and outstanding at September 30, 2017
1,289

 
$
20.64

 
2.4
 
$
32,985
Schedule of Compensation Cost for Share-based Payment Arrangements, Allocation of Share-based Compensation Costs by Plan
The following table sets forth the stock-based compensation expense recognized for the nine months ended September 30, 2017 and 2016.

 
For the Three Months Ended
For the Nine Months Ended
 
September 30,
September 30,
(in thousands)
2017
 
2016
2017
 
2016
RSUs
$
6,924

 
$
6,209

$
23,303

 
$
19,757

PBRSUs
1,400

 
3,766

4,877

 
8,312

Stock options
—

 
213

144

 
813

Employee stock purchase plan
294

 
352

1,234

 
977

 
$
8,618

 
$
10,540

$
29,558

 
$
29,859