Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities Available for Sale (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment Securities [Line Items]
Amortized Cost	$ 1,321,239	$ 1,169,613
Estimated Fair Value	1,291,063	1,171,303
US Government Agencies Residential Mortgage-backed Securities [Member]
Investment Securities [Line Items]
Amortized Cost	23,787	27,883
Estimated Fair Value	26,451	31,664
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investment Securities [Line Items]
Amortized Cost	972,500
Estimated Fair Value	$ 964,300

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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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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