Securities - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($) Securities	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Number of securities with unrealized losses | Securities	42
Securities pledged to municipalities for deposits in excess of FDIC limits, book value	$ 124.8
Securities pledged to municipalities for deposits in excess of FDIC limits, market value	129.6
Securities with market value sold under agreements to repurchase from various customers	40.3
Wholesale repurchase agreements with combined book value	6.7
Wholesale repurchase agreements with combined market value	6.9
Investment repurchase agreement, amount	$ 6.0
Repurchase agreement, maturity date	Sep. 18, 2016
Repurchase agreement, interest rate	4.36%
Securities with book value sold under agreements to repurchase from various customers	$ 39.7
Temporarily Impaired Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities with unrealized losses other than temporarily impaired	0	0