SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Unrealized Loss Position For:
Estimated Fair Value	$ 71,849,437	$ 75,652,396
Gross Unrealized Losses	(580,563)	(166,947)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated Fair Value	18,862,476	777,586
Gross Unrealized Losses	$ (439,060)	$ (2,030)
Number of Securities | security	381	30
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	$ 18,862,476	$ 777,586
Gross Unrealized Losses	$ (439,060)	$ (2,030)
Number of Securities | security	381	30
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	$ 0	$ 0
Gross Unrealized Losses	$ 0	$ 0
Number of Securities | security	0	0