Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Available for sale securities	$ 5,180,274	$ 5,525,015
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value
Assets:
Available for sale securities	5,180,274	5,525,015
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| Residential mortgage-backed securities
Assets:
Available for sale securities	4,923,211	5,265,204
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| States and political subdivisions
Assets:
Available for sale securities	235,910	238,675
Measured on a recurring basis: \| Assets/Liabilities Measured at Fair Value \| Other
Assets:
Available for sale securities	21,153	21,136
Measured on a recurring basis: \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Available for sale securities	21,153	21,136
Measured on a recurring basis: \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Other
Assets:
Available for sale securities	21,153	21,136
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2)
Assets:
Available for sale securities	5,128,424	5,471,019
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2) \| Residential mortgage-backed securities
Assets:
Available for sale securities	4,892,514	5,232,344
Measured on a recurring basis: \| Significant Other Observable Inputs (Level 2) \| States and political subdivisions
Assets:
Available for sale securities	235,910	238,675
Measured on a recurring basis: \| Significant Unobservable Inputs (Level 3)
Assets:
Available for sale securities	30,697	32,860
Measured on a recurring basis: \| Significant Unobservable Inputs (Level 3) \| Residential mortgage-backed securities
Assets:
Available for sale securities	$ 30,697	$ 32,860

- 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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