Basis of Presentation and Consolidation - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 135,501	$ 17,200	$ 135,501	$ 21,524	$ 21,491	$ 185,606
Preferred stock issued	0		0	0
Silver Creek [Member]
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 2,600		$ 2,600
Net cash proceeds from stock issuance			$ 2,599
Ownership interest percentage held by parent and its de facto agents	51.00%		51.00%
Silver Creek [Member] | Series C Preferred Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Preferred stock issued	400	1,400	400
Preferred stock price per share	$ 1.50	$ 1.50	$ 1.50
Net cash proceeds from stock issuance	$ 600	$ 2,000
Warrants issued to purchase preferred stock, shares	300	1,800	300	1,900
Silver Creek [Member] | Warrant Purchase Agreements [Member]
Summary Of Significant Accounting Policies [Line Items]
Warrants issued to purchase preferred stock, shares	1,300		1,300