Loss per Common Share - Computation of Basic and Diluted Net loss Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net loss	$ (21,692)	$ (20,402)	$ (365)	$ (19,452)	$ (27,714)	$ (30,794)	$ 6,631	$ (11,540)	$ (61,911)	$ (63,417)	$ (72,226)
Adjustments for basic net loss per common share:
Dividend accrued on redeemable preferred stock									(35,182)	(30,729)	(26,840)
Dividend requirements									(300)	(300)	(300)
Net loss attributable to common stockholders	$ (31,573)	$ (29,874)	$ (9,634)	$ (28,338)	$ (36,411)	$ (39,133)	$ (1,530)	$ (19,367)	$ (99,419)	$ (96,441)	$ (102,741)
Denominator:
Denominator for basic and diluted net loss per common share - adjusted weighted-average shares									28,200,983	28,039,625	27,490,717
Basic and diluted net loss per common share	$ (1.12)	$ (1.06)	$ (0.34)	$ (1.01)	$ (1.30)	$ (1.39)	$ (0.05)	$ (0.69)	$ (3.53)	$ (3.44)	$ (3.74)
Series E Redeemable Preferred Stock [Member]
Adjustments for basic net loss per common share:
Dividend accrued on redeemable preferred stock									$ (35,182)	$ (30,729)	$ (26,840)
Accretion of redeemable preferred stock									(2,026)	(1,995)	(3,375)
Series B Preferred Stock [Member]
Adjustments for basic net loss per common share:
Dividend requirements									(240)	(240)	(240)
Series D Preferred Stock [Member]
Adjustments for basic net loss per common share:
Dividend requirements									$ (60)	$ (60)	$ (60)