Redeemable Preferred Stock - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		12 Months Ended
	Nov. 07, 2018	Aug. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2018
Temporary Equity [Line Items]
Temporary equity, stock issued during period, shares, new issues			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
Preferred stock, dividend rate, percentage			9.00%	9.00%
Convertible Preferred Stock, Redemption Value			$ 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			$ 2,508		10,900	$ 3,134
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			$ 12,900
Redeemable preferred stock			148,426			$ 144,463
Convertible Preferred Stock Redemption Period Two [Member]
Temporary Equity [Line Items]
Convertible Preferred Stock, Redemption Value			$ 300,000
Convertible preferred stock, redemption value percent of stated value			200.00%
Convertible preferred stock, face value			$ 150,000
Common stock, shares issued upon conversion of preferred stock			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