Schedule III - Real Estate and Accumulated Depreciation (Real Estate Activity) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Beginning balance, January 1, 2014	$ 60,132
Additions during the year:
Acquisitions	176,519
Total additions	176,519nly_RealEstateAdditions
Deductions during the year:
Carrying value of real estate sold	23,270us-gaap_RealEstateCostOfRealEstateSold
Depreciation	3,349
Total deductions	26,619nly_RealEstateDeductions
Ending balance, December 31, 2014	$ 210,032

Amount of depreciation expense of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.FN4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Name:	us-gaap_SECScheduleIIIRealEstateAccumulatedDepreciationDepreciationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration