SECURITIES - Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Unrealized Loss Position For:
Estimated fair value	$ 79,357,596	$ 114,833,580
Gross unrealized losses	(486,219)	(291,441)
Agency Mortgage-Backed Securities
Unrealized Loss Position For:
Estimated fair value	290,197	19,007,519
Gross unrealized losses	$ (3,399)	$ (129,385)
Number of securities | security	22	491
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated fair value	$ 167,223	$ 7,388,239
Gross unrealized losses	$ (1,666)	$ (24,056)
Number of securities | security	12	139
Agency Mortgage-Backed Securities | Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated fair value	$ 122,974	$ 11,619,280
Gross unrealized losses	$ (1,733)	$ (105,329)
Number of securities | security	10	352