CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
Preferred stock ($0.000025 par value per share; 20,000,000 shares authorized; no shares issued and outstanding at September 30, 2018 and December 31, 2017)
Preferred stock, par value (in dollars per share)	$ 0.000025	$ 0.000025
Preferred stock, shares authorized (in shares)	20,000,000	20,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock and additional paid-in capital ($0.000025 par value per share; 750,000,000 shares authorized; 70,508,389 and 69,818,392 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively)
Common stock, par value (in dollars per share)	$ 0.000025	$ 0.000025
Common stock, shares authorized (in shares)	750,000,000	750,000,000
Common stock, shares issued (in shares)	70,079,747	70,596,559
Common stock, shares outstanding (in shares)	70,079,747	70,596,559