Summary of Significant Accounting Policies - Net Loss per Common Share and Stock-Based Compensation (Details)		1 Months Ended					6 Months Ended		12 Months Ended
	Jun. 20, 2018 shares	Sep. 30, 2018 USD ($) shares	Jun. 30, 2018 shares	Jun. 30, 2017 $ / shares shares	Feb. 28, 2017 $ / shares shares	Aug. 31, 2016 $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) item $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2015 $ / 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								13,878,000	17,380,000	14,290,000	11,919,000
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock								1,000	3,001,000	1,579,000	735,000
Shares issuable upon conversion of convertible notes at end of period (in shares)								23,878	501	501	23,878
Common stock equivalents under if-converted method for convertible notes (in shares)								23,878,000	501,000	501,000	718,000
Stock-Based Compensation
Number of employee share-based compensation plans | item									1
Weighted-Average Remaining Life (in years)
Outstanding at the end of the period								6 years 6 months 18 days	6 years 5 months 16 days	6 years 2 months 1 day	6 years 9 months 26 days	6 years 9 months 4 days
Vested or unvested and expected to vest at the end of the period								6 years 6 months 7 days	6 years 5 months 5 days	6 years 1 month 24 days	6 years 9 months 4 days
Exercisable at the end of the period								4 years 8 months 12 days	4 years 5 months 12 days	4 years 11 months 19 days	5 years 3 months 18 days
Aggregate Intrinsic Value
Outstanding at the end of the period | $							$ 13,513,000	$ 23,000	$ 5,818,000	$ 13,513,000
Vested or unvested and expected to vest at the end of the period | $							13,283,000	22,000	5,781,000	13,283,000
Exercisable at the end of the period | $							$ 3,733,000		$ 3,122,000	3,733,000
Additional disclosure for options
Aggregate number of common shares reserved for future issuance									24,700,000
Cash received for exercise of stock options | $									$ 4,301,000	650,000	$ 5,161,000
Stock compensation expense (reduction) | $								8,100,000	16,400,000	11,100,000	21,900,000
Estimated fair value of unvested employee awards, net of estimated forfeitures | $									$ 28,000,000
Weighted average vesting period of unvested employee awards									2 years
Total fair value of options vested | $								$ 17,121,000	$ 7,496,000	10,964,000	15,298,000
Total intrinsic value of options exercised | $									$ 3,787,000	$ 598,000	$ 3,142,000
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
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares									$ 3.55
Additional disclosure for options
Aggregate number of common shares reserved for future issuance			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%
Shares issued to participating employees									205,000
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)								65.63%	71.02%	67.34%	66.34%
Risk-free interest rate (as a percent)								1.29%	2.73%	2.00%	1.80%
Expected life								6 years 3 months 18 days	6 years	6 years	6 years 3 months 18 days
Weighted-average grant date fair value (in dollars per share) | $ / shares								$ 1.76	$ 6.70	$ 1.98	$ 8.91
Number of Stock Options
Outstanding at the beginning of the period (in shares)								11,813,000	11,971,000	13,679,000	9,689,000
Granted (in shares)								3,536,000	5,513,000	1,589,000	3,340,000
Exercised (in shares)									(742,000)	(191,000)	(555,000)
Forfeited/Canceled (in shares)								(1,670,000)	(1,178,000)	(3,106,000)	(661,000)
Outstanding at the end of the period (in shares)							11,971,000	13,679,000	15,564,000	11,971,000	11,813,000	9,689,000
Vested or unvested and expected to vest at the end of the period (in shares)							11,881,000	13,516,000	15,386,000	11,881,000	11,475,000
Exercisable at the end of the period (in shares)							7,996,000	7,898,000	8,405,000	7,996,000	6,453,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares								$ 13.03	$ 9.92	$ 10.70	$ 12.49
Granted (in dollars per share) | $ / shares								2.90	10.36	3.21	14.34
Exercised (in dollars per share) | $ / shares									4.67	3.42	9.30
Forfeited/Canceled (in dollars per share) | $ / shares								10.64	11.49	10.33	14.84
Outstanding at the end of the period (in dollars per share) | $ / shares							$ 9.92	10.70	10.20	9.92	13.03	$ 12.49
Vested or unvested and expected to vest at the end of the period (in dollars per share) | $ / shares							9.96	10.76	10.21	9.96	13.05
Exercisable at the end of the period (in dollars per share) | $ / shares							$ 12.16	$ 13.15	$ 11.47	$ 12.16	$ 12.63
Additional disclosure for options
Cash received for exercise of stock options | $									$ 3,500,000
Stock options | Chief Executive Officer
Additional disclosure for options
Stock compensation cost due to modification | $								$ 0	$ 116,000	$ 742,000	$ 3,100,000
Performance shares
Stock-Based Compensation
Vesting period		5 years
Number of vesting installments		2
Number of Stock Options
Granted (in shares)		295,200
Aggregate Intrinsic Value
Vested or unvested and expected to vest at the end of the period | $		$ 1,800,000
Restricted stock
Number of Restricted Stock
Unvested at the beginning of the period (in shares)								106,000	2,319,000	199,000	50,000
Awarded (in shares)								118,000		2,253,000	75,000
Vested (in shares)									(503,000)	(25,000)	(19,000)
Forfeited (in shares)								(25,000)		(108,000)
Unvested at the end of the period (in shares)							2,319,000	199,000	1,816,000	2,319,000	106,000	50,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares								$ 6.54	$ 2.82	$ 4.41	$ 9.23
Awarded (in dollars per share) | $ / shares								3.15		2.71	5.65
Vested (in dollars per share) | $ / shares									2.64	5.87	10.13
Forfeited (in dollars per share) | $ / shares								7.52		2.68
Unvested at the end of the period (in dollars per share) | $ / shares							$ 2.82	$ 4.41	$ 2.87	$ 2.82	$ 6.54	$ 9.23
Restricted stock | Officers
Stock-Based Compensation
Number of equal installments restrictions lapse				3	3	3
Service period				5 years	5 years	5 years
Number of Restricted Stock
Awarded (in shares)				239,000	529,830	117,800
Forfeited (in shares)							(71,380)
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares				$ 4.71	$ 2.47	$ 3.15
Additional disclosure for options
Estimated fair value of unvested employee awards, net of estimated forfeitures | $									$ 2,600,000
Deferred share units
Additional disclosure for options
Stock compensation expense (reduction) | $								$ 215,000	$ 361,000	$ 206,000	$ 380,000
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									1
2016 Plan | Stock options | Maximum
Stock-Based Compensation
Vesting period									4 years
Exercise period									10 years
2018 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	7,500,000
Stock-Based Compensation
Common stock authorized for issuance (in shares)	19,500,000
2018 Plan | Stock options
Number of Stock Options
Exercised (in shares)									(742,000)