Mortgage-Backed Securities - Non-Agency Securities Having Deteriorated Credit When Acquired (Detail) - Non-Agency RMBS Having Deteriorated Credit When Acquired - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Outstanding principal balance:
Beginning of period	$ 3,138,265	$ 3,550,698
End of period	2,673,350	3,138,265
Amortized cost:
Beginning of period	1,695,079	1,958,726
End of period	$ 1,381,839	$ 1,695,079

Investments, Amortized Cost on Loans Held-in-portfolio [Roll Forward]

No definition available.

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- Definition

Principal Amount Outstanding of Loans Held-in-portfolio [Roll Forward]

+ References

No definition available.

+ Details

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- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and other-than-temporary impairments), for certain contractual rights to receive money on demand or on fixed or determinable dates. This relates to loans accounted for as available-for-sale debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82894507&loc=d3e13503-111538

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

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- Definition

The undiscounted sum of all amounts, including amounts deemed principal, interest, fees, penalties, and other under the loan, owed to the investor at the reporting date, whether or not currently due and whether or not any such amounts have been written or charged off by the investor. This relates to loans accounted for as available-for-sale debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

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