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CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares
Dec. 31, 2017
Dec. 31, 2016
Stockholders' equity:    
Common stock, par value (in dollars per share) $ 0.001 $ 0.001
Common stock, shares authorized (in shares) 300,000,000 300,000,000
Common stock, shares issued (in shares) 124,976,373 121,636,273
Common stock, shares outstanding (in shares) 124,976,373 121,636,273
1 Percent Convertible Senior Notes [Member]    
LIABILITIES AND STOCKHOLDERS' EQUITY    
Interest rate on convertible senior notes 1.00% 1.00%
2 3/4 Percent Convertible Senior Notes [Member]    
LIABILITIES AND STOCKHOLDERS' EQUITY    
Interest rate on convertible senior notes 2.75% 2.75%