Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity

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Stockholders' Equity
6 Months Ended
Jun. 30, 2019
Equity [Abstract]  
Stockholders' Equity Stockholders’ Equity
We grant options and awards to employees and non-employee directors pursuant to a stockholder approved stock incentive plan, which is described in further detail in Note 8, Stockholders' Equity, of Notes to Consolidated Financial Statements in our 2018 Annual Report.

The following is a summary of our stock option and restricted stock activity and related information:
Stock Options Restricted Stock Awards
Shares Weighted-Average Exercise Price Shares Weighted-Average Grant Date Fair Value
Balance as of December 31, 2018 1,736,304  $ 66.71  132,273  $ 130.63 
Granted 311,884  $ 117.63  98,404  $ 114.15 
Options exercised/RSUs vested (104,249) $ 23.16  (69,529) $ 110.87 
Balance as of June 30, 2019 1,943,939  $ 77.17  161,148  $ 129.10 

As of June 30, 2019, outstanding options to purchase 1.3 million shares were exercisable with a weighted average exercise price per share of $55.57.
Employee Stock Purchase Plan

The price at which common stock is purchased under the Amended Employee Stock Purchase Plan, or ESPP, is equal to 85% of the fair market value of the common stock on the first or last day of the offering period, whichever is lower. As of June 30, 2019, 60,642 shares were available for future purchases under the ESPP.