Cash, Cash Equivalents, and Restricted Cash - Additional Information (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 6,389,000	$ 10,425,000
Held In Cash Money Market Account [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	6,000,000	10,000,000
Held In Cash Collateral Accounts [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash	$ 389,000	$ 425,000