Investment Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) position	Dec. 31, 2014 USD ($) position	Dec. 31, 2013 USD ($)
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	7.00%
General obligation bonds as percentage of tax-exempt municipal securities that were not pre-refunded	42.00%
Special revenue bonds as percentage of tax exempt municipal securities that were not pre-refunded	58.00%
Maximum percentage of any general obligation bonds in one state	11.00%
Percentage of tax-exempt securities insured by bond insurers	6.00%
Residential and commercial mortgage-backed securities, percentage of collateral consisting of prime loans	99.00%
Percentage of corporate securities associated with financial services industry	25.00%	21.00%
Other-than-temporary impairments | $	$ 0	$ 0	$ 0
Number of positions | position	690	2,000
Residential Mortgage-backed securities
Investment [Line Items]
Mortgage back securities as percent of agency mortgage-backed securities	98.00%	99.00%