Collateral Positions (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Repurchase agreements collateral pledged	$ 15,368,586,000	$ 15,523,415,000
Securities loaned collateral pledged	1,918,895,000	1,927,496,000
Interest rate swaps, futures contracts and currency forward contracts collateral pledged	89,700,000	109,900,000
Total collateral pledged	17,382,908,000	17,561,431,000
Cash	27,553,000	7,327,000
Total collateral held	27,553,000	7,327,000
Agency RMBS pledged related to unsettled sales	189,263,000	0
Cash
Derivative [Line Items]
Securities loaned collateral pledged	2,000,000	0
Total collateral pledged	7,775,000	620,000
Interest Rate Swaps, Future Contracts and Currency Forward Contracts
Derivative [Line Items]
Interest rate swaps, futures contracts and currency forward contracts collateral pledged	95,427,000	110,520,000
Interest Rate Swaps, Future Contracts and Currency Forward Contracts \| Cash
Derivative [Line Items]
Interest rate swaps, futures contracts and currency forward contracts collateral pledged	5,775,000	620,000
Agency RMBS
Derivative [Line Items]
Repurchase agreements collateral pledged	11,546,380,000	11,788,765,000
Securities loaned collateral pledged	638,181,000	623,181,000
Agency RMBS \| Interest Rate Swaps, Future Contracts and Currency Forward Contracts
Derivative [Line Items]
Interest rate swaps, futures contracts and currency forward contracts collateral pledged	89,652,000	109,900,000
Agency RMBS \| Interest Rate Swaps, Future Contracts and Currency Forward Contracts \| Immaterial Error
Derivative [Line Items]
Interest rate swaps, futures contracts and currency forward contracts collateral pledged		86,200,000
CMBS
Derivative [Line Items]
Repurchase agreements collateral pledged	1,856,165,000	1,737,831,000
Securities loaned collateral pledged	1,278,714,000	1,304,315,000
Non-Agency RMBS
Derivative [Line Items]
Repurchase agreements collateral pledged	1,110,185,000	1,143,373,000
GSE CRT
Derivative [Line Items]
Repurchase agreements collateral pledged	855,856,000	853,446,000
MBS and GSE CRT Securities
Derivative [Line Items]
Total collateral pledged	$ 17,375,133,000	$ 17,560,811,000

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Total Collateral Pledged, at Fair Value

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Fair value of securities received as collateral against derivative assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226013-175313

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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)
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The carrying amount, as of the date of the latest financial statement presented, of finance receivables which are owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and for which the transferee is not permitted to sell or re-pledge them to an unrelated party.

+ References

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

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Fair value of the securities pledged as collateral against securities loaned.

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Fair value of securities pledged as collateral against securities sold under agreement to repurchase.

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