Cash, Cash Equivalents, and Restricted Cash - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 6,779,000	$ 6,389,000
Held In Cash Money Market Account [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	6,000,000	6,000,000
Held In Cash Collateral Accounts [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 779,000	$ 389,000