Investment Securities - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Securities pledged as collateral	435,000,000		$ 470,400,000
Proceeds from sale of investment securities	117,300,000	$ 170,400,000
Gains from sale of investment securities	1,000,000	979,000
Losses from sale of investment securities	1,000	$ 0
Unamortized premiums	19,600,000		22,400,000
Unaccreted discounts	234,000		188,000
Held-to-maturity securities, unrealized loss			0
Owned investment securities	194,925,000		330,754,000
Total unrealized losses on securities	$ 1,809,000		3,594,000
Weighted average duration of MBS portfolio	4 years 3 months 18 days
Mortgage-Backed Securities ("MBS") [Member]
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0		$ 0

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity in a continuous loss position.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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The cash inflow associated with the sale of debt and equity securities classified as available-for-sale securities.

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The carrying value as of the balance sheet date of the assets received as collateral against securities loaned to other broker-dealers. Borrowers of securities generally are required to provide collateral to the lenders of securities, commonly cash but sometimes other securities or standby letters of credit, with a value slightly higher than that of the securities borrowed. In instances where the entity is permitted to sell or re-pledge these securities, the entity reports the fair value of the collateral received and the related obligation to return the collateral as a liability.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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Available For Sale Securities Unamortized Discount

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Available For Sale Securities Unamortized Premium

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Weighted Average Remaining Expected Life Of Residential Mortgage Backed Securities Available For Sale One

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