Securities - Additional Information (Detail) (USD $)	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Schedule Of Marketable Securities [Line Items]
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	98.80%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	78.50%
Carrying value of pledged securities	$ 2,500,000,000	$ 2,700,000,000
Securities reclassified from available for sale to held to maturity		2,300,000,000
Available for sale securities reclassified to held to maturity, unrealized gain		165,700,000
Available for sale securities reclassified to held to maturity, unrealized gain net of tax		107,700,000
Net unrealized gain on transferred securities included in accumulated other comprehensive income before Tax	156,600,000
Net unrealized gain on transferred securities included in accumulated other comprehensive income after Tax	$ 101,800,000

Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period.

+ References+ Details

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.

+ References+ Details