NET INCOME (LOSS) PER COMMON SHARE - Schedule of Net Income (Loss) per Share Reconciliation (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income (loss)	$ (889,772)	$ (2,163,091)	$ 54,148
Net income (loss) attributable to noncontrolling interests	1,391	(226)	(260)
Net income (loss) attributable to Annaly	(891,163)	(2,162,865)	54,408
Dividends on preferred stock	142,036	136,576	129,312
Net income (loss) available (related) to common stockholders	$ (1,033,199)	$ (2,299,441)	$ (74,904)
Weighted average shares of common stock outstanding-basic (in shares)	1,414,659,439	1,434,912,682	1,209,601,809
Add: Effect of stock awards, if dilutive (in shares)	0	0	0
Weighted average shares of common stock outstanding-diluted (in shares)	1,414,659,439	1,434,912,682	1,209,601,809
Net income (loss) per share available (related) to common share
Basic (in dollars per share)	$ (0.73)	$ (1.60)	$ (0.06)
Diluted (in dollars per share)	$ (0.73)	$ (1.60)	$ (0.06)