Fair Value of Financial Instruments - Additional Information (Detail)	9 Months Ended	12 Months Ended
Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Fair Value Of Financial Instruments [Line Items]
Cash and cash equivalents	$ 30,173,000	$ 71,157,000
Restricted cash	316,000	$ 527,000
Short-term investments	$ 19,885,000
Fair value of note payable to related party, description	The fair value measurement of debt instruments is based on a discounted cash flow model and is sensitive to the change in yield (Level 3 in the fair value hierarchy
Embedded derivatives	$ 0
MidCap Credit Facility
Fair Value Of Financial Instruments [Line Items]
Fair value of note payable to related party, description	MidCap credit facility - If the yield increases by approximately 1% from 11.5% to 12.5%, the fair value of the notes payable would change from $38.1 million to $37.1 million, or a decrease of $1.0 million or 2.7%; if the yield decreases 1% from 11.5% to 10.5%, the fair value of the note payable with conversion feature would change from $38.1 million to $39.1 million, or an increase of $1.0 million or 2.6%. If the yield increases by approximately 2% from 11.5% to 13.5%, the fair value of the note payable with the conversion feature would change from $38.1 million to $36.2 million, or a decrease of $1.9 million or 5.0%; if the yield decreases 2% from 11.5% to 9.5%, the fair value of the note payable with conversion feature would change from $38.1 million to $40.2 million, or an increase of $2.1 million or 5.5%.
Mann Group Notes
Fair Value Of Financial Instruments [Line Items]
Fair value of note payable to related party, description	Mann Group Notes — If the yield increases by approximately 2% from 31% to 33%, the fair value of the notes payable would change from $42.0 million to $40.1 million, or a decrease of $1.9 million or 4.5%; if the yield decreases 2% from 31% to 29%, the fair value of the note payable with conversion feature would change from $42.0 million to $44.1 million, or an increase of $2.1 million or 5.0%. If the yield increases by approximately 4% from 31% to 35%, the fair value of the note payable with the conversion feature would change from $42.0 million to $38.4 million, or a decrease of $3.6 million or 8.6%; if the yield decreases 4% from 31% to 27%, the fair value of the note payable with conversion feature would change from $42.0 million to $46.4 million, or an increase of $4.4 million or 10.5%.
Senior Convertible Notes
Fair Value Of Financial Instruments [Line Items]
Fair value of note payable to related party, description	Senior Convertible Notes – If the yield increases by approximately 2% from 31% to 33%, the fair value of the notes payable would change from $7.3 million to $7.1 million, or a decrease of $0.2 million and 2.7%; if the yield decreases 2% from 31% to 29%, the fair value of the note payable with conversion feature would change from 7.3 million to $7.5 million, or an increase of $0.2 million or 2.7%. If the yield increases by approximately 4% from 31% to 35%, the fair value of the note payable with the conversion feature would change from $7.3 million to $7.0 million, or a decrease of $0.3 million or 4.1%; if the yield decreases 4% from 31% to 27%, the fair value of the note payable with conversion feature would change from $7.3 million to $7.7 million, or an increase of 0.4 million and 5.5%.
Note Payable Yield Changes 1 \| MidCap Credit Facility
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	1.00%
Note payable, percentage of yield	0.125	0.115
Note payable to related party, fair value	$ 37,100,000	$ 38,100,000
Change in fair value of note payable value due to change in interest rate percentage	$ 1,000,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	2.70%
Note Payable Yield Changes 1 \| Mann Group Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)		2.00%
Note payable, percentage of yield	0.33	0.31
Note payable to related party, fair value	$ 40,100,000	$ 42,000,000
Change in fair value of note payable value due to change in interest rate percentage	$ 1,900,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	4.50%
Note Payable Yield Changes 1 \| Senior Convertible Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	2.00%
Note payable, percentage of yield	0.33	0.31
Note payable to related party, fair value	$ 7,100,000	$ 7,300,000
Change in fair value of note payable value due to change in interest rate percentage	$ 200,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	2.70%
Note Payable Yield Changes 2 \| MidCap Credit Facility
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	1.00%
Note payable, percentage of yield	0.105	0.115
Note payable to related party, fair value	$ 39,100,000	$ 38,100,000
Change in fair value of note payable value due to change in interest rate percentage	$ 1,000,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	2.60%
Note Payable Yield Changes 2 \| Mann Group Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	2.00%
Note payable, percentage of yield	0.29	0.31
Note payable to related party, fair value	$ 44,100,000	$ 42,000,000
Change in fair value of note payable value due to change in interest rate percentage	$ 2,100,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	5.00%
Note Payable Yield Changes 2 \| Senior Convertible Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	2.00%
Note payable, percentage of yield	0.29	0.31
Note payable to related party, fair value	$ 7,500,000	$ 7,300,000
Change in fair value of note payable value due to change in interest rate percentage	$ 200,000	$ 200,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	2.70%	2.70%
Note Payable Yield Changes 3 \| MidCap Credit Facility
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	2.00%
Note payable, percentage of yield	0.135	0.115
Note payable to related party, fair value	$ 36,200,000	$ 38,100,000
Change in fair value of note payable value due to change in interest rate percentage	$ 1,900,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	5.00%
Note Payable Yield Changes 3 \| Mann Group Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	4.00%
Note payable, percentage of yield	0.35	0.31
Note payable to related party, fair value	$ 38,400,000	$ 42,000,000
Change in fair value of note payable value due to change in interest rate percentage	$ 3,600,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	8.60%
Note Payable Yield Changes 3 \| Senior Convertible Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	4.00%
Note payable, percentage of yield	0.35	0.31
Note payable to related party, fair value	$ 7,000,000	$ 7,300,000
Change in fair value of note payable value due to change in interest rate percentage	$ 300,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	4.10%
Note Payable Yield Changes 4 \| MidCap Credit Facility
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	2.00%
Note payable, percentage of yield	0.095	0.115
Note payable to related party, fair value	$ 40,200,000	$ 38,100,000
Change in fair value of note payable value due to change in interest rate percentage	$ 2,100,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	5.50%
Note Payable Yield Changes 4 \| Mann Group Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	4.00%
Note payable, percentage of yield	0.27	0.31
Note payable to related party, fair value	$ 46,400,000	$ 42,000,000
Change in fair value of note payable value due to change in interest rate percentage	$ 4,400,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	10.50%
Note Payable Yield Changes 4 \| Senior Convertible Notes
Fair Value Of Financial Instruments [Line Items]
Note payable, percentage of interest rate increases (decreases)	4.00%
Note payable, percentage of yield	0.27	0.31
Note payable to related party, fair value	$ 7,700,000	$ 7,300,000
Change in fair value of note payable value due to change in interest rate percentage	$ 400,000
Increase (decrease) in fair value of note payable percentage due to increases (decreases) in interest rate percentage	5.50%
Money Market Funds
Fair Value Of Financial Instruments [Line Items]
Cash and cash equivalents	$ 30,200,000	71,200,000
Restricted cash	300,000	$ 500,000
U.S. Treasury bills or notes.
Fair Value Of Financial Instruments [Line Items]
Short-term investments	$ 19,900,000

Fair Value of Financial Instruments - Additional Information (Detail)

9 Months Ended

12 Months Ended

Sep. 30, 2019

USD ($)

Dec. 31, 2018

USD ($)

Fair Value Of Financial Instruments [Line Items]

Cash and cash equivalents

$ 30,173,000

$ 71,157,000

Restricted cash

316,000

$ 527,000

Short-term investments

$ 19,885,000

Fair value of note payable to related party, description

The fair value measurement of debt instruments is based on a discounted cash flow model and is sensitive to the change in yield (Level 3 in the fair value hierarchy

Embedded derivatives

$ 0

MidCap Credit Facility

Fair Value Of Financial Instruments [Line Items]

Fair value of note payable to related party, description

MidCap credit facility - If the yield increases by approximately 1% from 11.5% to 12.5%, the fair value of the notes payable would change from $38.1 million to $37.1 million, or a decrease of $1.0 million or 2.7%; if the yield decreases 1% from 11.5% to 10.5%, the fair value of the note payable with conversion feature would change from $38.1 million to $39.1 million, or an increase of $1.0 million or 2.6%. If the yield increases by approximately 2% from 11.5% to 13.5%, the fair value of the note payable with the conversion feature would change from $38.1 million to $36.2 million, or a decrease of $1.9 million or 5.0%; if the yield decreases 2% from 11.5% to 9.5%, the fair value of the note payable with conversion feature would change from $38.1 million to $40.2 million, or an increase of $2.1 million or 5.5%.

Mann Group Notes

Fair Value Of Financial Instruments [Line Items]

Fair value of note payable to related party, description

Mann Group Notes — If the yield increases by approximately 2% from 31% to 33%, the fair value of the notes payable would change from $42.0 million to $40.1 million, or a decrease of $1.9 million or 4.5%; if the yield decreases 2% from 31% to 29%, the fair value of the note payable with conversion feature would change from $42.0 million to $44.1 million, or an increase of $2.1 million or 5.0%. If the yield increases by approximately 4% from 31% to 35%, the fair value of the note payable with the conversion feature would change from $42.0 million to $38.4 million, or a decrease of $3.6 million or 8.6%; if the yield decreases 4% from 31% to 27%, the fair value of the note payable with conversion feature would change from $42.0 million to $46.4 million, or an increase of $4.4 million or 10.5%.

Senior Convertible Notes

Fair Value Of Financial Instruments [Line Items]

Fair value of note payable to related party, description

Senior Convertible Notes – If the yield increases by approximately 2% from 31% to 33%, the fair value of the notes payable would change from $7.3 million to $7.1 million, or a decrease of $0.2 million and 2.7%; if the yield decreases 2% from 31% to 29%, the fair value of the note payable with conversion feature would change from 7.3 million to $7.5 million, or an increase of $0.2 million or 2.7%. If the yield increases by approximately 4% from 31% to 35%, the fair value of the note payable with the conversion feature would change from $7.3 million to $7.0 million, or a decrease of $0.3 million or 4.1%; if the yield decreases 4% from 31% to 27%, the fair value of the note payable with conversion feature would change from $7.3 million to $7.7 million, or an increase of 0.4 million and 5.5%.