Class A Redeemable Convertible Preferred Stock (Detail) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended			9 Months Ended
	Jun. 25, 2018	Jun. 24, 2018	Dec. 22, 2016	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 30, 2015
Temporary Equity [Line Items]
Redemption of preferred stock							$ 0	$ 50,096
Gain on redemption of preferred stock				$ 0	$ 0	$ 9,075	$ 0	$ 9,075
Series A Redeemable Convertible Preferred Stock
Temporary Equity [Line Items]
Temporary equity, shares issued														14,666,667
Temporary equity, face value (in dollars per share)			$ 7.50											$ 7.50
Temporary equity, liquidation value			$ 49,500											$ 110,000
Temporary equity shares redemption (in shares)			8,066,667
Temporary equity redemption amount			$ 55,000
Temporary equity, redemption per share (in dollars per share)	$ 7.30		$ 6.82
Redemption discounts percentage	4.55%		9.09%
Temporary equity, shares outstanding				0	0	0	0	0	0	0	0	6,864,000	6,864,000
Redemption of preferred stock	$ 50,100
Additional amount per share added for accrued and unpaid dividends (in dollars per share)	$ 0.14
Gain on redemption of preferred stock		$ 9,100				$ 0
Temporary equity, carrying amount		37,800
Temporary equity, recognized amount of beneficial conversion feature		$ (21,300)
Temporary equity, fair value			$ 32,900											$ 73,200
Temporary equity issued, price per share (in dollars per share)			$ 4.99											$ 4.99
Temporary equity issue discount			$ 16,600											$ 36,800
Temporary equity, value of beneficial conversion feature			$ 20,100											$ 44,700
Temporary equity, beneficial conversion feature, recognition period (in years)							3 years
Series A Redeemable Convertible Preferred Stock | Payment In Kind Period Two
Temporary Equity [Line Items]
Temporary equity, shares issued													264,000