FAIR VALUE - Fair Value and Related Disclosures for Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Residential Mortgage Securities Classified as Held-to-Maturity [Member]
Held-to-maturity securities disclosure [Abstract]
Held-to-maturities, Amortized Cost Basis	$ 1,306	$ 1,706
Held-to-maturities, Gross Unrealized Gains	7	17
Held-to-maturities, Fair Value	1,313	1,723
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	10,057,786	10,529,646
Available-for-sale securities, Gross Unrealized Gains	75,587	85,740
Available-for-sale securities, Gross Unrealized Losses	79,699	37,022
Available-for-sale securities, Fair Value	10,053,674	10,578,364
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	2,971,627	2,875,637
Available-for-sale securities, Gross Unrealized Gains	9,485	8,958
Available-for-sale securities, Gross Unrealized Losses	20,694	12,432
Available-for-sale securities, Fair Value	$ 2,960,418	$ 2,872,163

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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