Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities (Parenthetical) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities transferred from available-for-sale to held-to-maturity, net unrealized gain	$ 3.1	$ 3.6

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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