Stockholders' Equity - Early Exercise of Stock Options (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares subject to repurchase (in shares)	244,658	137,831	597,000	199,895	244,658	137,831
Number of Shares
Beginning balance (in shares)	244,658	137,831	597,000
Early exercise of options (in shares)	117,000	238,000	2,000
Vested (in shares)	(162,000)	(120,000)	(461,000)
Repurchased (in shares)	0	(11,000)	0
Ending balance (in shares)	199,895	244,658	137,831
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Other long-term liabilities				$ 0.9	$ 1.0	$ 0.3