Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Estimated Fair Market Value	$ 318,608	$ 7,368
Mortgage-backed securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	8,868,311	9,020,099
Gross Unrealized Gains	171,997	150,310
Gross Unrealized Losses	(546)	(19)
Estimated Fair Market Value	9,039,762	9,170,390
Mortgage-backed securities [Member] \| GNMA pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	1,117,983	1,139,894
Gross Unrealized Gains	30,420	26,353
Gross Unrealized Losses	(71)	(19)
Estimated Fair Market Value	1,148,332	1,166,228
Mortgage-backed securities [Member] \| FNMA pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	4,285,217	4,407,970
Gross Unrealized Gains	70,876	60,059
Gross Unrealized Losses	(303)
Estimated Fair Market Value	4,355,790	4,468,029
Mortgage-backed securities [Member] \| FHLMC pass-through certificates [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	3,389,925	3,390,467
Gross Unrealized Gains	68,329	61,689
Gross Unrealized Losses	(172)
Estimated Fair Market Value	3,458,082	3,452,156
Mortgage-backed securities [Member] \| FHLMC and FNMA - REMICs [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	75,186	81,768
Gross Unrealized Gains	2,372	2,209
Gross Unrealized Losses
Estimated Fair Market Value	77,558	83,977
Investment Securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	314,433	6,767
Gross Unrealized Gains	4,175	601
Gross Unrealized Losses
Estimated Fair Market Value	318,608	7,368
Investment Securities [Member] \| Equity Securities [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	6,813	6,767
Gross Unrealized Gains	587	601
Gross Unrealized Losses
Estimated Fair Market Value	7,400	7,368
Investment Securities [Member] \| Corporate debt [Member]
Amortized cost and estimated fair market value of investment securities and mortgage-backed securities available for sale
Amortized Cost	307,620
Gross Unrealized Gains	3,588
Gross Unrealized Losses
Estimated Fair Market Value	$ 311,208

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

This item represents the gross unrealized losses for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized losses are the excess of the carrying value of the Available-for-sale Securities over their fair value as of the reporting date. Such gross unrealized losses are included in other comprehensive income in the statement 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 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.

+ References+ Details