Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	$ 9,350,970	$ 9,170,390
Total available for sale equity securities	7,400	7,368
Total available for sale securities	9,358,370	9,177,758
Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	9,039,762	9,170,390
Financial Services Industry [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale equity securities	7,400	7,368
Corporate debt [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	311,208
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale equity securities	7,400	7,368
Total available for sale securities	7,400	7,368
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Financial Services Industry [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale equity securities	7,400	7,368
Significant Other Observable Inputs (Level 2) [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	9,350,970	9,170,390
Total available for sale securities	9,350,970	9,170,390
Significant Other Observable Inputs (Level 2) [Member] \| Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	9,039,762	9,170,390
Significant Other Observable Inputs (Level 2) [Member] \| Corporate debt [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities	311,208
Significant Unobservable Inputs (Level 3) [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities
Total available for sale equity securities
Total available for sale securities
Significant Unobservable Inputs (Level 3) [Member] \| Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities
Significant Unobservable Inputs (Level 3) [Member] \| Financial Services Industry [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale equity securities
Significant Unobservable Inputs (Level 3) [Member] \| Corporate debt [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Total available for sale debt securities

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading. Such securities are reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

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For an unclassified balance sheet, total of debt securities categorized neither as held-to-maturity nor trading. Such securities are reported at fair value; unrealized gains and losses of such securities are excluded from earnings and included in other comprehensive income, a separate component of shareholders' equity, unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

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- Definition

For an unclassified balance sheet, this item represents equity securities categorized neither as held-to-maturity nor trading. Equity securities represent ownership interests or the right to acquire ownership interests in corporations and other legal entities which ownership interest is represented by shares of common or preferred stock (which is not mandatorily redeemable or redeemable at the option of the holder), convertible securities, stock rights, or stock warrants. Unrealized gains and losses related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

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