Accretion of Debt Issuance Cost and Debt Discount (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Accretion expense - debt issuance cost	$ 31	$ 35	$ 35
Accretion expense - debt discount	$ 1,700	$ 1,722	$ 1,553