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Fair Value Measurements (Narrative) (Detail) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2014
Sep. 30, 2014
Sep. 30, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Fair value assets, Level 1 to Level 2 transfers amount $ 0 $ 0 $ 0
Fair value assets, Level 2 to Level 1 transfers amount 0 0 $ 0
Non-Agency Mortgage-Backed Securities      
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Transfers into Level 3 0 0  
Transfers out of Level 3 $ 0 $ 0