Lease Commitments And Contingencies (Detail) (USD $) In Thousands	Jun. 30, 2011
Lease Commitments and Contingencies [Line Items]
Lease Commitment - 2011	$ 1,501
Lease Commitment - 2012	3,005
Lease Commitment - 2013	3,006
Lease Commitment - 2014	2,522
Lease Commitment - 2015	161
Lease Commitment - Later years	27
Aggregate future minimum lease payments	10,222
Sublease income - 2011	85
Sublease income - 2012	70
Aggregate future minimum sublease income	155
Net Lease Commitment - 2011	1,416
Net Lease Commitment - 2012	2,935
Net Lease Commitment - 2013	3,006
Net Lease Commitment - 2014	2,522
Net Lease Commitment - 2015	161
Net Lease Commitment - Later years	27
Aggregate future net minimum lease payments	$ 10,067

Lease Commitments and Contingencies [Line Items]

Name:	nly_LeaseCommitmentsAndContingenciesLineItems
Namespace Prefix:	nly
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDue
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, net required rental payments due within the remaining current year of the balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueCurrentRemaining
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, net required rental payments due in the fourth calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueFourYearsCalendarYear
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, net required rental payments due in the first calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueOneYearCalendarYear
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, net required rental payments due after the fifth calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueThereafterCalendarYear
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, net required rental payments due in the third calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueThreeYearsCalendarYear
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, net required rental payments due in the second calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_NetOperatingLeasesFutureMinimumPaymentsDueTwoYearsCalendarYear
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 within the remaining current year of the balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueCurrentRemaining
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 fourth calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueFourYearsCalendarYear
Namespace Prefix:	nly
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractually required future rental payments receivable on noncancelable subleasing arrangements in the remaining current year.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsCurrentRemaining
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 first calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentalsOneYearCalendarYear
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.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueOneYearCalendarYear
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.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueThereafterCalendarYear
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 third calendar year following the current balance sheet date relating to leases defined as operating.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueThreeYearsCalendarYear
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.

+ Details

Name:	nly_OperatingLeasesFutureMinimumPaymentsDueTwoYearsCalendarYear
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

+ Details

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	us-gaap_OperatingLeasesFutureMinimumPaymentsDueFutureMinimumSubleaseRentals
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant