Income Taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Securities transactions income tax expense	$ 574,000	$ 147,000	$ 1,720,000
Recognition of interest and penalties related to unrecognized tax benefits	$ 0	$ 0	$ 0
Earliest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2012
Latest Tax Year [Member]
Income Taxes [Line Items]
Year under examination	2014