CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
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)	140,339,615	137,928,828
Common stock, shares outstanding (in shares)	140,339,615	137,928,828
0.125 Percent Convertible Senior Notes [Member]
LIABILITIES AND STOCKHOLDERS' EQUITY
Interest rate on convertible senior notes	0.125%	0.125%
1 Percent Convertible Senior Notes [Member]
LIABILITIES AND STOCKHOLDERS' EQUITY
Interest rate on convertible senior notes	1.00%	1.00%

Face amount or stated value per share of common stock.

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Contractual interest rate for funds borrowed, under the debt agreement.

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