Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Securities transactions income tax (benefit) expense	$ (21,000)	$ 3,200,000	$ 574,000
Recognition of interest and penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
Earliest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2014
Latest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2016