Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity

Stockholders' Equity
3 Months Ended
Mar. 31, 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 283,387  118.04  60,857  115.83 
Options exercised/RSUs vested (92,036) 21.26  (66,078) 106.42 
Balance as of March 31, 2019 1,927,655  $ 76.43  127,052  $ 136.13 

As of March 31, 2019, outstanding options to purchase 1.3 million shares were exercisable with a weighted average exercise price per share of $51.36.

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 March 31, 2019, 64,008 shares were available for future purchases under the ESPP.