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Commercial Mortgage Residual Interests (Schedule of Key Assumptions Used in Determining the Value of the Residuals) (Details)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Mortgage Loans On Real Estate [Line Items]    
Discount rate 20.00%nnn_DiscountRateValuationAdjustment 20.00%nnn_DiscountRateValuationAdjustment
Loss severity of loans in foreclosure 20.00%nnn_LossSeverityOfLoansInForeclosure 20.00%nnn_LossSeverityOfLoansInForeclosure
LIBOR Forward 3-month curve Forward 3-month curve
Prime Forward curve Forward curve
Minimum    
Mortgage Loans On Real Estate [Line Items]    
Average Life Equivalent CPR Speeds Range 0.87%nnn_AverageLifeEquivalentCprSpeedsRange
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 [1] 0.80%nnn_AverageLifeEquivalentCprSpeedsRange
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 [1]
Foreclosures Frequency Curve Default Model 0.70%nnn_ForeclosuresFrequencyCurveDefaultModel
/ us-gaap_RangeAxis
= us-gaap_MinimumMember		 0.07%nnn_ForeclosuresFrequencyCurveDefaultModel
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
Maximum    
Mortgage Loans On Real Estate [Line Items]    
Average Life Equivalent CPR Speeds Range 26.30%nnn_AverageLifeEquivalentCprSpeedsRange
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 [1] 20.76%nnn_AverageLifeEquivalentCprSpeedsRange
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 [1]
Foreclosures Frequency Curve Default Model 2.45%nnn_ForeclosuresFrequencyCurveDefaultModel
/ us-gaap_RangeAxis
= us-gaap_MaximumMember		 2.43%nnn_ForeclosuresFrequencyCurveDefaultModel
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
[1] Conditional prepayment rate