Stockholders' Equity - Additional Information (Detail) $ in Thousands, shares in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2018 USD ($) shares	Mar. 31, 2018 USD ($) shares	Mar. 31, 2017 USD ($)
Class of Stock [Line Items]
Conversion ratio		1
Fees and commissions		$ 526	$ 135
Over-allotment Option
Class of Stock [Line Items]
Aggregate purchase price, net of offering costs	$ 14,500
Option to purchase additional shares of common stock | shares	1.5
Fees and commissions	$ 500
Convertible Preferred Stock
Class of Stock [Line Items]
Convertible preferred stock, shares issued | shares		6.1
Discharged amounts outstanding		$ 22,650
Amounts to be invoiced		27,349
Maximum | At-the-Market Program
Class of Stock [Line Items]
Aggregate purchase price, net of offering costs		$ 189,700