Mortgage-Backed Securities (Schedule Of Mortgage Backed Securities Portfolio Backed By U.S. Government Agency Or U.S. Government Sponsored Entity) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	$ 17,269,221	$ 9,424,904
Gross unrealized losses	(2,067)	(33,125)
Gross unrealized gains	347,220	195,437
Carrying value/estimated fair value	17,614,374	9,587,216
Fannie Mae Certificates [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	12,132,959	6,537,472
Gross unrealized losses	(1,216)	(18,342)
Gross unrealized gains	256,361	138,626
Carrying value/estimated fair value	12,388,104	6,657,756
Fannie Mae Certificates ARMS [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	11,427,141	6,537,472
Gross unrealized losses	(172)	(18,342)
Gross unrealized gains	255,762	138,626
Carrying value/estimated fair value	11,682,731	6,657,756
Fannie Mae Certificates Fixed Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	705,818	0
Gross unrealized losses	(1,044)	0
Gross unrealized gains	599	0
Carrying value/estimated fair value	705,373	0
Freddie Mac Certificates [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	5,136,262	2,887,432
Gross unrealized losses	(851)	(14,783)
Gross unrealized gains	90,859	56,811
Carrying value/estimated fair value	5,226,270	2,929,460
Freddie Mac Certificates ARMS [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	4,710,834	2,887,432
Gross unrealized losses	(83)	(14,783)
Gross unrealized gains	90,263	56,811
Carrying value/estimated fair value	4,801,014	2,929,460
Freddie Mac Fixed Rate [Member]
Schedule of Available-for-sale Securities [Line Items]
MBS Amortized Cost	425,428	0
Gross unrealized losses	(768)	0
Gross unrealized gains	596	0
Carrying value/estimated fair value	$ 425,256	$ 0

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13, 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

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- Definition

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6475986&loc=d3e42546-110969

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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