Redeemable Preferred Shares - Additional Information (Detail) - USD ($)				9 Months Ended
	Aug. 30, 2017	May 22, 2017	Mar. 22, 2017	Sep. 30, 2018	Oct. 01, 2017	Dec. 31, 2017
Temporary Equity [Line Items]
Proceeds from issuance of redeemable preferred shares			$ 150,000,000		$ 150,000,000
Temporary equity, par value				$ 0.01		$ 0.01
Convertible Preferred Stock, Redemption Value				$ 300,000,000
Issuance costs		$ 3,700,000			$ 3,737,000
Proceeds from issuance of redeemable preferred shares, net of issuance costs		146,300,000
Embedded derivative liability		10,900,000		8,161,000		$ 4,685,000
Adjusted proceeds from issuance of redeemable preferred shares		$ 135,500,000
Adjusted redeemable preferred stock				157,600,000
Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Accretion value				22,100,000
Convertible Preferred Stock Redemption Period One [Member]
Temporary Equity [Line Items]
Convertible Preferred Stock, Redemption Value				$ 262,500,000
Convertible preferred stock, redemption value percent of stated value				175.00%
Convertible Preferred Stock Redemption Period Two [Member]
Temporary Equity [Line Items]
Convertible Preferred Stock, Redemption Value				$ 300,000,000
Convertible preferred stock, redemption value percent of stated value				200.00%
Convertible preferred stock, total shares issued upon conversion				5,300,000
Debt Instrument Redemption Period One [Member]
Temporary Equity [Line Items]
Redemption period				7 years
Embedded derivative liability [Member]
Temporary Equity [Line Items]
Redemption per share				$ 56.324
Convertible Preferred Stock, Redemption Value				$ 300,000,000
Stock price per share		$ 19.05
Debt instrument face value				$ 150,000,000
Series A Redeemable Preferred Stock [Member]
Temporary Equity [Line Items]
Temporary equity, stock issued during period, shares, new issues			140,202
Temporary equity, liquidation preference per share			$ 1,000
Temporary equity, par value			0.01
Redemption per share			$ 28.162
Preferred stock, dividend rate, percentage			9.00%
Percentage of temporary equity stock issued during period to common stock outstanding			19.99%
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
Conversion of series B redeemable preferred stock into series A redeemable preferred stock	9,798