Redeemable Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		9 Months Ended		12 Months Ended
	Nov. 07, 2018	Aug. 30, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2017	Dec. 31, 2019
Temporary Equity [Line Items]
Temporary equity, stock issued during period, shares, new issues			150,000		150,000		150,000	150,000
Proceeds from issuance of redeemable preferred shares							$ 150,000
Temporary equity, par value			$ 0.01		$ 0.01		$ 0.01	$ 0.01
Preferred stock, dividend rate, percentage			9.00%	9.00%	9.00%	9.00%
Convertible Preferred Stock, Redemption Value			$ 300,000		$ 300,000
Preferred stock redemption extended date	Sep. 14, 2025				Sep. 14, 2025
Issuance costs							$ 3,700
Proceeds from issuance of redeemable preferred shares, net of issuance costs							146,300
Embedded derivative liability							10,900
Adjusted proceeds from issuance of redeemable preferred shares							$ 135,500
Redemption period					7 years
Carrying value of redeemable preferred stock	$ 17,200
Accumulated accretion value net of modification adjustment					$ 39,100
Redeemable preferred stock			174,597		174,597			$ 160,980
Convertible Preferred Stock Redemption Period Two [Member]
Temporary Equity [Line Items]
Convertible Preferred Stock, Redemption Value			$ 300,000		$ 300,000
Convertible preferred stock, redemption value percent of stated value			200.00%		200.00%
Convertible preferred stock, face value			$ 150,000		$ 150,000
Common stock, shares issued upon conversion of preferred stock			5,300,000		5,300,000
Series A Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Temporary equity, stock issued during period, shares, new issues							140,202
Temporary equity, par value		$ 0.01
Temporary equity, liquidation preference per share		1,000
Temporary equity, conversion price		$ 28.162
Preferred stock, dividend rate, percentage		9.00%
Convertible preferred stock, threshold stock price trigger							$ 84.49
Series B Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Temporary equity, stock issued during period, shares, new issues							9,798