Summary of Significant Accounting Policies - Net Loss per Common Share and Stock-Based Compensation (Details)			1 Months Ended				3 Months Ended		6 Months Ended	9 Months Ended
	Jul. 01, 2017 shares	Dec. 09, 2016 shares	Jun. 30, 2018 shares	Jun. 30, 2017 item $ / shares shares	Feb. 28, 2017 item $ / shares shares	Aug. 31, 2016 item $ / shares shares	Sep. 30, 2018 USD ($) item $ / shares shares	Sep. 30, 2017 USD ($) item $ / shares shares	Jun. 30, 2018 $ / shares shares	Sep. 30, 2018 USD ($) item $ / shares shares	Sep. 30, 2017 USD ($) item $ / shares shares
Computation of Net Loss per Common Share
Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock at end of period							18,153,000	15,360,000		18,153,000	15,360,000
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock							3,153,000	2,570,000		3,378,000	1,210,000
Shares issuable upon conversion of convertible notes at end of period (in shares) | item							501	740		501	740
Common stock equivalents under if-converted method for convertible notes (in shares)							501,000	18,685,000		501,000	22,128,000
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares										$ 3.55
Additional disclosure for options
Cash received for exercise of stock options | $										$ 2,943,000	$ 363,000
Stock compensation expense (reduction) | $										0
Estimated fair value of unvested employee awards, net of estimated forfeitures | $							$ 33,400,000			$ 33,400,000
Weighted average vesting period of unvested employee awards										2 years 6 months
ESPP
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)										0.00%
Volatility (as a percent)										70.10%
Risk-free interest rate (as a percent)										2.14%
Expected life										6 months
Additional disclosure for options
Aggregate number of common shares reserved for future issuance			1,000,000						1,000,000
Threshold period of continuous employment per year			3 months
Threshold period of customary employment in calendar year			5 months
Threshold period of customary employment per week			140 days
Purchase price of common stock (as a Percent)			85.00%
Plan Purchase Period			6 months
Maximum Percentage Of Periodic Payroll Deductions (as a percent)			15.00%
Estimated subscriptions outstanding							120,000			120,000
Stock compensation expense (reduction) | $							$ 213,000			$ 213,000
Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)							0.00%	0.00%		0.00%	0.00%
Volatility (as a percent)							71.91%	69.31%		70.99%	67.26%
Risk-free interest rate (as a percent)							2.89%	1.93%		2.72%	2.00%
Expected life							6 years	6 years		6 years	6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares							$ 6.11	$ 4.13		$ 6.74	$ 1.91
Number of Stock Options
Outstanding at the beginning of the period (in shares)									11,971,000	11,971,000
Granted (in shares)										5,431,000
Exercised (in shares)										(595,000)
Forfeited/Canceled (in shares)										(590,000)
Outstanding at the end of the period (in shares)							16,217,000			16,217,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares									$ 9.92	$ 9.92
Granted (in dollars per share) | $ / shares										10.43
Exercised (in dollars per share) | $ / shares										4.94
Forfeited/Canceled (in dollars per share) | $ / shares										11.40
Outstanding at the end of the period (in dollars per share) | $ / shares							$ 11.47			$ 11.47
Additional disclosure for options
Cash received for exercise of stock options | $										$ 2,900,000
Stock options | Minimum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares										$ 1.84
Stock options | Maximum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares										$ 12.21
Performance Shares [Member]
Stock-Based Compensation
Exercise period										5 years
Number of equal installments restrictions lapse										2
Number of Restricted Stock
Awarded (in shares)										295,200
Restricted stock
Stock-Based Compensation
Exercise period				5 years	5 years	5 years
Number of equal installments restrictions lapse | item				3	3	3
Number of Restricted Stock
Unvested at the beginning of the period (in shares)									2,319,000	2,319,000
Vested (in shares)										(503,000)
Forfeited (in shares)	(71,380)
Unvested at the end of the period (in shares)							1,816,000			1,816,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares									$ 2.82	$ 2.82
Vested (in dollars per share) | $ / shares										2.64
Unvested at the end of the period (in dollars per share) | $ / shares							$ 2.87			$ 2.87
Additional disclosure for options
Stock compensation expense (reduction) | $							$ 4,300,000	$ 2,600,000		$ 12,000,000	$ 8,300,000
Restricted stock | Officers
Number of Restricted Stock
Awarded (in shares)				239,000	529,830	117,800
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares				$ 4.71	$ 2.47	$ 3.15
Additional disclosure for options
Stock compensation expense (reduction) | $										$ 0
2016 Plan
Computation of Net Loss per Common Share
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock		19,500,000
Stock-Based Compensation
Common stock authorized for issuance (in shares)		7,500,000
2016 Plan | Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Number of group of awards for which expected term is calculated for and applied | item										1
2016 Plan | Stock options | Maximum
Stock-Based Compensation
Vesting period										4 years
Exercise period										10 years