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Commercial Mortgage Loan Portfolio - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Mortgage Loans on Real Estate [Line Items]                      
Provision for loan losses $ 0 $ 0 $ 15,000,000 $ 0 $ 0 $ 0 $ 0 $ 0 $ 15,000,000 $ 0 $ 0
Commercial Mortgage Portfolio Segment [Member]                      
Mortgage Loans on Real Estate [Line Items]                      
Provision for loan losses                 $ 10,000,000 $ 0  
Terminal capitalization rate                 11.00%    
Discount Rate                 10.00%    
Multifamily - Williston, ND [Member] | Commercial Mortgage Portfolio Segment [Member]                      
Mortgage Loans on Real Estate [Line Items]                      
Provision for loan losses                 $ 10,000,000