Investment Securities - Schedule of Maturities of Investment Securities Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 817,243	$ 886,031
Available-for-Sale, Fair Value	829,341	907,498
Within one year [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost	18,567	18,544
Available-for-Sale, Fair Value	18,647	18,658
After one year but within five years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost	29,547	28,855
Available-for-Sale, Fair Value	29,771	29,034
After five years but within ten years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost	276,659	321,103
Available-for-Sale, Fair Value	281,321	329,580
After ten years [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost	492,470	517,529
Available-for-Sale, Fair Value	$ 499,602	$ 530,226

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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