FAIR VALUE - Fair Value and Related Disclosures for Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Residential Mortgage Securities Classified as Held-to-Maturity [Member]
Held-to-maturity securities disclosure [Abstract]
Held-to-maturities, Amortized Cost Basis	$ 922	$ 998
Held-to-maturities, Gross Unrealized Gains	2	2
Held-to-maturities, Fair Value	924	1,000
Agency Securities Classified as Available-for-sale [Member] \| Fannie Mae/Freddie Mac [Member]
Available-for-sale securities disclosure Items [Abstract]
Available-for-sale securities, Amortized Cost Basis	7,423,367	8,890,949
Available-for-sale securities, Gross Unrealized Gains	110,895	64,593
Available-for-sale securities, Gross Unrealized Losses	15,154	23,753
Available-for-sale securities, Fair Value	7,519,108	8,931,789
Agency Securities Classified as Available-for-sale [Member] \| Ginnie Mae [Member]
Available-for-sale securities disclosure Items [Abstract]
Available-for-sale securities, Amortized Cost Basis	792,060	2,284,331
Available-for-sale securities, Gross Unrealized Gains	15,994	11,560
Available-for-sale securities, Gross Unrealized Losses	715	7,133
Available-for-sale securities, Fair Value	$ 807,339	$ 2,288,758

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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