Investment Securities - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Percentage of debt securities considered to be of investment-grade	97.00%		97.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	7.00%		7.00%
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded			35.00%
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded			65.00%
Maximum percentage of any general obligation bonds in one state			11.00%
Percentage of tax-exempt securities insured by bond insurers	17.00%		17.00%
Percentage of corporate securities associated with financial services industry	23.00%		23.00%	21.00%
Other-than-temporary impairments	$ 0	$ 0