FAIR VALUE - Fair Value and Related Disclosures for Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 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	$ 1,392	$ 1,706
Held-to-maturities, Gross Unrealized Gains	14	17
Held-to-maturities, Fair Value	1,406	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	10,207,636	10,529,646
Available-for-sale securities, Gross Unrealized Gains	79,710	85,740
Available-for-sale securities, Gross Unrealized Losses	68,746	37,022
Available-for-sale securities, Fair Value	10,218,600	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	2,840,066	2,875,637
Available-for-sale securities, Gross Unrealized Gains	9,628	8,958
Available-for-sale securities, Gross Unrealized Losses	19,401	12,432
Available-for-sale securities, Fair Value	$ 2,830,293	$ 2,872,163

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount before other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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