ACQUISITION OF HATTERAS - Aggregate Consideration and Preliminary Fair Value of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands		12 Months Ended
	Jul. 12, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Assets:
Bargain purchase gain		$ 0	$ 72,576	$ 0
Hatteras
Consideration Transferred:
Cash	$ 521,082
Preferred shares:
Total consideration	1,806,289
Net Assets:
Cash	562,780
Principal receivable	438,005
Accrued interest and dividend receivable	83,814
Other assets	57,250
Total assets acquired	12,846,633
Securitized debt of consolidated VIEs	54,135
Dividends payable	670
Payable for investments purchased	2,643
Accrued interest payable	4,833
Accounts payable and other liabilities	97,039
Total liabilities assumed	10,967,768
Net assets acquired	1,878,865
Bargain purchase gain	72,576
Hatteras | Repurchase agreements
Net Assets:
Financial liabilities	10,422,757
Hatteras | Credit risk transfer securities, at fair value
Net Assets:
Financial assets	116,770
Hatteras | Residential mortgage loans
Net Assets:
Financial assets	360,447
Hatteras | Mortgage servicing rights
Net Assets:
Financial assets	355,820
Hatteras | Other secured financing
Net Assets:
Financial liabilities	35,769
Hatteras | Other derivatives, at fair value
Net Assets:
Financial assets	8,677
Financial liabilities	349,922
Hatteras | Agency mortgage-backed securities, at fair value
Net Assets:
Financial assets	10,863,070
Hatteras | Common Stock
Consideration Transferred:
Common equity	997,707
Preferred shares:
Preferred stock	997,707
Hatteras | Preferred Stock
Consideration Transferred:
Common equity	287,500
Preferred shares:
Exchange of Hatteras preferred stock for Annaly preferred stock	278,252
Preferred stock fair value adjustment	9,248
Preferred stock	$ 287,500