Stock-Based Compensation - Stock Plans (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Stock Plans [Abstract]
Shares of common stock added	101,509,223	101,509,223		100,993,173
RSUs [Member]
Stock Plans [Abstract]
Unrecognized compensation expense related to RSUs	$ 18.9	$ 18.9
Weighted average amortization period		1 year 7 months 6 days
Non-vested Stock Options [Member]
Stock Plans [Abstract]
Unrecognized compensation expense related to non-vested stock options	$ 53.2	$ 53.2
Weighted average amortization period		1 year 6 months
2015 Equity Incentive Plan [Member]
Stock Plans [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Other Share Increase (Decrease)	5,000,000
Number of shares authorized for issuance pursuant to stock awards (in shares)	23,500,000	23,500,000
Number of shares available for future grant (in shares)	6,607,438	6,607,438
2015 Equity Incentive Plan [Member] | Stock Options [Member]
Stock Plans [Abstract]
Options outstanding (in shares)	10,388,835	10,388,835
Options exercisable (in shares)	3,569,106	3,569,106
2015 Equity Incentive Plan [Member] | RSUs [Member]
Stock Plans [Abstract]
Restricted stock units outstanding (in shares)	1,502,975	1,502,975
2017 Employee Stock Purchase Plan [Member]
Stock Plans [Abstract]
Number of shares available for future grant (in shares)	1,893,951	1,893,951
Shares of common stock added			500,000
Purchase price of common stock, percent		85.00%
Number of shares of common stock reserved for issuance (in shares)	2,000,000	2,000,000
Shares issued under ESPP (in shares)	0	34,774
Accrued compensation for contributions to employee stock purchase plan	$ 0.3	$ 0.3