FAIR VALUE - Fair Value and Related Disclosures for Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 966	$ 998
Held-to-maturities, Gross Unrealized Gains	2	2
Held-to-maturities, Fair Value	968	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	7,560,352	8,890,949
Available-for-sale securities, Gross Unrealized Gains	73,230	64,593
Available-for-sale securities, Gross Unrealized Losses	45,315	23,753
Available-for-sale securities, Fair Value	7,588,267	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	895,906	2,284,331
Available-for-sale securities, Gross Unrealized Gains	19,084	11,560
Available-for-sale securities, Gross Unrealized Losses	1,643	7,133
Available-for-sale securities, Fair Value	$ 913,347	$ 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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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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