Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Option Activity
A summary of the option activity during the three months ended March 31, 2018 follows:    
 
 
Options Outstanding
 
 
Shares
 
Weighted Average Exercise Price Per Share
 
Weighted Average Remaining Contractual Term
 
Aggregate Intrinsic Value
 
 
 
 
 
 
(in years)
 
(in thousands)
Balances at December 31, 2017
 
3,017,113

 
$
10.13

 
 
 
 
Options exercised
 
(216,308
)
 
11.52

 
 
 
 
Balances at March 31, 2018
 
2,800,805

 
$
10.03

 
4.09
 
$
198,276

Vested and expected to vest at March 31, 2018
 
2,800,805

 
$
10.03

 
4.09
 
$
198,276

Exercisable at March 31, 2018
 
2,753,930

 
$
9.26

 
4.01
 
$
197,059
Summary of RSU Activity
A summary of the RSU activity during the three months ended March 31, 2018 follows:
 
 
Number of Shares Underlying Outstanding RSUs
 
Weighted-Average Grant-Date Fair Value per RSU
Non-Vested outstanding at December 31, 2017
 
7,178,015

 
$
62.79

RSUs granted
 
2,376,440

 
83.32

RSUs vested
 
(1,222,641
)
 
65.24

RSUs forfeited
 
(238,327
)
 
63.38

Non-Vested outstanding at March 31, 2018
 
8,093,487

 
$
68.43
Summary of Shares Available for Grant
The summary of shares available for issuance of equity-based awards (including stock options, RSUs and shares issuable under our 2013 ESPP) during the three months ended March 31, 2018 follows:
 
 
Shares Available for Grant
 
 
2013 Plan
 
2013 ESPP
Balances at December 31, 2017
 
7,207,291

 
3,666,392

Authorized
 
4,023,117

 
804,623

Granted
 
(2,376,440
)
 
—

Forfeited
 
238,327

 
—

Balances at March 31, 2018
 
9,092,295

 
4,471,015