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Securities - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Amortized Cost and Fair Value Debt Securities [Abstract]      
Proceeds from sales of securities available for sale $ 6.6 $ 1.5 $ 0.2
Net carrying amount of securities held to maturity 2.6    
Realized gain on securities held to maturity 0.2    
Securities pledged to secure public deposits and securities sold under agreements to repurchase $ 3,500.0 $ 3,200.0