Investment Securities - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Percentage of debt securities considered to be of investment-grade	98.00%		98.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	9.10%		9.10%
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded			39.00%
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded			61.00%
Maximum percentage of any general obligation bonds in one state			14.00%
Percentage of corporate securities associated with financial services industry	25.00%		25.00%	21.00%
Other-than-temporary impairments	$ 0	$ 0