Fair Value Disclosures of Financial Assets (Narrative) (Detail) (USD $)	9 Months Ended
Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale of securities, fair value	$ 910,055,000	$ 859,362,000
Minimum discount rate on appraisals of collateral securing the loan	10.00%
Maximum discount rate on appraisals of collateral securing the loan	50.00%
Collateral for collateral dependent loans	47,300,000	75,300,000
Valuation allowance on impaired loans	2,900,000	4,700,000
Federal Agency Debentures [Member] \| AAA Rating [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale of securities, fair value	51,000,000
Federal Agency M B S [Member] \| AAA Rating [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale of securities, fair value	850,400,000
Municipal Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale of securities, fair value	$ 3,100,000

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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