Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity

Stockholders' Equity
9 Months Ended
Sep. 30, 2018
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 2017 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, 2017  1,876,332  $ 53.17  133,294  $ 91.60 
Granted  224,312  161.83  62,033  169.91 
Options exercised/RSUs vested  (318,653) 58.64  (60,135) 83.95 
Forfeited  (12,228) 104.54  (1,165) 125.16 
Balance as of September 30, 2018  1,769,763  $ 65.60  134,027  $ 130.99 

As of September 30, 2018, outstanding options to purchase 1.3 million shares were exercisable with a weighted average exercise price per share of $43.98.

Employee Stock Purchase Plan

The price at which common stock is purchased under the Amended ESPP is equal to 0.85% of the fair market value of the common stock on the first or last day of the offering period, whichever is lower. As of September 30, 2018, 65,007 shares were available for future purchases under the Amended ESPP.