Stock‑Based Compensation (Details)	1 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) period shares	Dec. 31, 2015 USD ($) $ / shares shares
2015 Plan
Stock Based Compensation
Shares reserved for issuance	3,451,495	3,743,382
Shares reserved for issuance as a proportion common stock outstanding (as a percent)	4.00%
Automatic termination of plan, term		10 years
2015 Plan | Non-employee directors
Stock Based Compensation
Aggregate grant date fair value of awards granted, annual ceiling limitation amount | $	$ 500,000
2015 Plan | Newly appointed non-employee directors
Stock Based Compensation
Aggregate grant date fair value of awards granted, annual ceiling limitation amount | $	$ 1,000,000
2015 Plan | Minimum
Stock Based Compensation
Shares reserved for issuance	3,500,000
2007 Plan
Stock Based Compensation
Shares reserved for issuance		15,258,947
Automatic termination of plan, term		10 years
2015 Employee Stock Purchase Plan
Stock Based Compensation
Shares reserved for issuance	893,548
Shares reserved for issuance as a proportion common stock outstanding (as a percent)	1.00%
Price in relation to fair market value of common stock on the date of grant, lower range limit (as a percent)	85.00%
Maximum amount of award or purchase during a calendar year | $	$ 25,000
Requisite service requirement (in hours per week)	20 hours
Requisite service requirement (in months per year)	5 months
Maximum offering period, term	27 months
Number of expected offering periods each year | period	2
Offering period, expected term	6 months
Maximum employee contribution of employee's cash compensation (as a percent)	15.00%
Maximum aggregate amount of shares a participant may receive during the fiscal year | $	$ 5,000
2015 Employee Stock Purchase Plan | Minimum
Stock Based Compensation
Shares reserved for issuance	880,000
Employee and non-employee stock options | 2015 Plan
Stock Based Compensation
Price in relation to fair market value of common stock on the date of grant, lower range limit (as a percent)	100.00%
Vesting period	4 years
Expiration period	10 years
Maximum number of shares a participant may receive during the fiscal year (in shares)	1,725,000
Additional number of shares a new employee may receive during initial fiscal year of employment (in shares)	350,000
Employee and non-employee stock options | 2007 Plan
Stock Based Compensation
Price in relation to fair market value of common stock on the date of grant, lower range limit (as a percent)		100.00%
Vesting period		4 years
Expiration period		10 years
Stock options, exercise price range, number outstanding (in shares)		8,621,395
Stock options, exercise price range, lower range limit (in dollars per share) | $ / shares		$ 0.0065
Stock options, exercise price range, upper range limit (in dollars per share) | $ / shares		12.8501
Stock options, exercise price range, weighted-average exercise price (in dollars per share) | $ / shares		$ 3.27
Shares exercised and unvested		1,300,000
Period after Market Stand-off repurchase right expires		210 days
Accelerated stock-based compensation expense | $		$ 1,300,000
Stock appreciation rights | 2015 Plan
Stock Based Compensation
Price in relation to fair market value of common stock on the date of grant, lower range limit (as a percent)	100.00%
Vesting period	4 years
Expiration period	10 years
Maximum number of shares a participant may receive during the fiscal year (in shares)	1,725,000
Additional number of shares a new employee may receive during initial fiscal year of employment (in shares)	350,000
Restricted shares and stock Units | 2015 Plan
Stock Based Compensation
Maximum number of shares a participant may receive during the fiscal year (in shares)	1,500,000
Additional number of shares a new employee may receive during initial fiscal year of employment (in shares)	300,000
Performance awards | 2015 Plan
Stock Based Compensation
Maximum amount of award or purchase during a calendar year | $	$ 5,000,000