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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Issuance of shares of Common Stock upon exercise of stock options and restricted stock units 1,624,376 3,012,937 2,068,226
Issuance of shares of Common Stock under the ESPP 233,712 157,277 126,648
Issuance of shares of Common Stock for services rendered 4,905 2,532 3,438
Issuance of shares of Common Stock   4,945,000  
Convertible Senior Notes 1.25 Percent Due 2020      
Issuance of shares of Common Stock upon conversion of convertible senior notes   7,095,350 77
Convertible Senior Notes 0.375 Percent Due 2018      
Issuance of shares of Common Stock upon conversion of convertible senior notes 148,761 7,201,058 114