Goodwill and Other Intangible Assets - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) ReportingUnit	Dec. 31, 2016 USD ($) ReportingUnit	Dec. 31, 2015 USD ($)
Goodwill And Intangible Assets [Line Items]
Number of reporting units with carrying amount of zero or less than zero | ReportingUnit	0	0
Carrying amount of reporting unit	$ 0	$ 0
Amortization of intangible assets	422,000	69,000	$ 54,000
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Carrying amount of reporting unit	$ 0	$ 0