Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Securities transactions income tax expense (benefit)	$ 1.4	$ (21,000)	$ 3,200,000
Change in income tax expense (benefit) due to write down of net deferred tax asset resulting from federal tax rate change	$ 4,300,000
Federal tax rate, percentage	21.00%	35.00%
Recognition of interest and penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
Earliest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2015
Latest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2017