SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized Loss Position For:
Estimated fair value	$ 76,098,985	$ 114,833,580
Gross unrealized losses	(213,751)	(291,441)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	1,154,762	19,007,519
Gross unrealized losses	$ (9,176)	$ (129,385)
Number of securities | security	23	491
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated fair value	$ 1,154,762	$ 7,388,239
Gross unrealized losses	$ (9,176)	$ (24,056)
Number of securities | security	23	139
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated fair value	$ 0	$ 11,619,280
Gross unrealized losses	$ 0	$ (105,329)
Number of securities | security	0	352