PLEDGED ASSETS - Assets Pledged to Counterparties (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	$ 3,712,561,000	$ 2,972,290,000
Accrued interest receivable	14,444,000	11,512,000
Restricted cash	32,349,000	20,950,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	13,962,000	10,623,000
Restricted cash	25,296,000	10,835,000
Total Assets Pledged To Counterparties	3,751,819,000	2,981,715,000
Clearing Margin [Member]
Pledged Assets [Line Items]
Accrued interest receivable		266,000
Restricted cash		0
Total Assets Pledged To Counterparties		11,234,000
Derivative [Member]
Pledged Assets [Line Items]
Accrued interest receivable	0	0
Restricted cash	7,053,000	9,865,000
Total Assets Pledged To Counterparties	7,053,000	9,865,000
Insurance Capital [Member]
Pledged Assets [Line Items]
Accrued interest receivable		4,000
Restricted cash		250,000
Total Assets Pledged To Counterparties		1,319,000
Total Pledged Financial Instruments [Member]
Pledged Assets [Line Items]
Accrued interest receivable	13,962,000	10,893,000
Restricted cash	32,349,000	20,950,000
Total Assets Pledged To Counterparties	3,758,872,000	3,004,133,000
Residential Mortgage Backed Securities [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	3,712,561,000	2,972,290,000
Residential Mortgage Backed Securities [Member] \| Repurchase Agreement [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	3,612,244,000	2,854,062,000
Residential Mortgage Backed Securities [Member] \| Repurchase Agreement [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	100,317,000	106,195,000
Residential Mortgage Backed Securities [Member] \| Clearing Margin [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0
Residential Mortgage Backed Securities [Member] \| Clearing Margin [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		10,968,000
Residential Mortgage Backed Securities [Member] \| Derivative [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	0	0
Residential Mortgage Backed Securities [Member] \| Derivative [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	0	0
Residential Mortgage Backed Securities [Member] \| Insurance Capital [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		1,065,000
Residential Mortgage Backed Securities [Member] \| Insurance Capital [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value		0
Residential Mortgage Backed Securities [Member] \| Total Pledged Financial Instruments [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	3,612,244,000	2,855,127,000
Residential Mortgage Backed Securities [Member] \| Total Pledged Financial Instruments [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	$ 100,317,000	$ 117,163,000

Total securities and cash pledged as collateral.

No definition available.

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- References

No definition available.

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- Definition

The fair value as of the balance sheet date of financial instruments and other positions owned by the entity and pledged to counterparties as collateral, including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=98513485&loc=SL98516268-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

Reference 4: 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(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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