FAIR VALUE - Fair Value and Related Disclosures for Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Residential Mortgage Securities Classified as Held-to-Maturity [Member]
Held-to-maturity securities disclosure [Abstract]
Held-to-maturities, Amortized Cost Basis	$ 1,099	$ 1,134
Held-to-maturities, Gross Unrealized Gains	3	3
Held-to-maturities, Fair Value	1,102	1,137
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	9,105,307	8,949,793
Available-for-sale securities, Gross Unrealized Gains	65,623	56,041
Available-for-sale securities, Gross Unrealized Losses	52,284	74,276
Available-for-sale securities, Fair Value	9,118,646	8,931,558
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	3,104,734	3,040,275
Available-for-sale securities, Gross Unrealized Gains	14,638	8,681
Available-for-sale securities, Gross Unrealized Losses	11,747	17,692
Available-for-sale securities, Fair Value	$ 3,107,625	$ 3,031,264

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 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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