Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Percentage of debt securities considered to be of investment-grade	96.00%		96.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	5.00%		5.00%
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			11.00%
Percentage of tax-exempt securities insured by bond insurers	16.00%		16.00%
Residential and commercial mortgage-backed securities, percentage of collateral consisting of prime loans	99.00%		99.00%
Percentage of corporate securities associated with financial services industry	20.00%		20.00%		23.00%
Other-than-temporary impairments	$ 0	$ 0	$ 0	$ 0