Investment Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014 Investment	Dec. 31, 2013 Investment
Investments, Debt and Equity Securities [Abstract]
Carrying value of securities pledged to secure public deposits and for other purposes as required by law	$ 230.9	$ 271.1
Number of investments in non-federal issuer representing more than 10% of shareholders' equity	0	0
Maximum investment in any single non-federal issuer representing shareholders' equity	10.00%	10.00%