Credit Risk and Non-Agency Securities - Additional Information (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Credit Risk and Non-Agency Securities \| Credit Risk and Non-Agency Securities \| Portfolio Concentration Risk
Debt and Equity Securities, FV-NI [Line Items]
Percentage of portfolio invested	11.80%	11.50%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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