Lease Commitments And Contingencies (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Lease Commitments and Contingencies [Line Items]
Lease Commitment - 2012	$ 3,003
Lease Commitment - 2013	3,004
Lease Commitment - 2014	2,522
Lease Commitment - 2015	161
Lease Commitment - 2016	27
Lease Commitment - Later years
Aggregate future minimum lease payments	8,717
Sublease income - 2012	70
Sublease income - 2013
Sublease income - 2014
Sublease income - 2015
Sublease income - 2016
Sublease income - Later years
Aggregate future minimum sublease income	70
Net Lease Commitment - 2012	2,933
Net Lease Commitment - 2013	3,004
Net Lease Commitment - 2014	2,522
Net Lease Commitment - 2015	161
Net Lease Commitment - 2016	27
Net Lease Commitment - Later years
Aggregate future net minimum lease payments	$ 8,647

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The total of future contractually required net payments on leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due in the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due in the fourth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due in the first calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due after the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due in the third calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, net required rental payments due in the second calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due in the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due in the fourth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the first calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the fourth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the third calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the second calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due in the first calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due after the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements after fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due in the third calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

For leases having an initial or remaining non-cancelable letter-terms in excess of one year, required rental payments due in the second calendar year following the current balance sheet date relating to leases defined as operating.

+ References

No definition available.

+ Details

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Namespace Prefix:	nly_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The total of future contractually required payments on leases defined as operating.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 122

-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 13

-Paragraph 16

-Subparagraph b(i)

-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 840

-SubTopic 10

-Section 55

-Paragraph 40

-Subparagraph (Note 3)

-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

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- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 13

-Paragraph 16

-Subparagraph b(ii)

-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 Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

+ Details

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