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Mortgage-Backed Securities Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2016 | Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract] |  |
Weighted Average Holdings | $ 1,225 | $ 1,323

X
- Definition Weighted average holdings for the period, based on amortized cost. + References No definition available. + Details
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Balance Type: | debit
Period Type: | duration

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