Investment in Silver Creek - Schedule of Non-Controlling Interest in Subsidiary (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Minority Interest [Line Items]
Balance at the beginning of the period	$ (1,539)	$ 239
Net loss attributable to Silver Creek	(1,160)	(1,778)	$ 170
Balance at date of deconsolidation	(1,852)
Carrying value of non-controlling interest	1,852
Balance at the end of the period		(1,539)	$ 239
Series C Preferred Stock [Member]
Minority Interest [Line Items]
Issuance of Silver Creek Series C preferred stock	847	(827)
Silver Creek [Member]
Minority Interest [Line Items]
Net loss attributable to Silver Creek	(1,160)	$ (951)
Carrying value of non-controlling interest	$ 1,852