Investment Securities - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2016 USD ($) position	Mar. 31, 2015 USD ($)	Dec. 31, 2015 position
Investment [Line Items]
Percentage of debt securities considered to be of investment-grade	98.00%
Percentage of tax-exempt municipal securities classified as pre-refunded	6.00%
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded	46.00%
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded	54.00%
Percentage of tax exempt securities	5.40%
Maximum percentage of any general obligation bonds in one state	9.00%
Percentage of corporate securities associated with financial services industry	24.00%		25.00%
Securities in unrealized loss positions, number of positions	360
Number of positions			2,000
Other-than-temporary impairments | $	$ 0	$ 0
Residential Mortgage-Backed Securities
Investment [Line Items]
Mortgage-backed Securities as Percentage of Agency Mortgage-backed Securities	98.00%		98.00%