SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Unrealized Loss Position For:
Estimated fair value	$ 114,833,580	$ 92,623,788
Gross unrealized losses	(291,441)	(2,609,491)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	19,007,519	65,552,879
Gross unrealized losses	$ (129,385)	$ (2,285,609)
Number of securities | security	491	2,189
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months | Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	$ 7,388,239	$ 22,418,036
Gross unrealized losses	$ (24,056)	$ (432,352)
Number of securities | security	139	713
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months | Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	$ 11,619,280	$ 43,134,843
Gross unrealized losses	$ (105,329)	$ (1,853,257)
Number of securities | security	352	1,476