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Consolidations and Transfers of Financial Assets Transfers of Financial Assets (Details) - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Transfers of Financial Assets [Line Items]      
Portfolio securitizations: unpaid principal balance $ 252.7 $ 264.7 $ 212.0
Unconsolidated VIEs [Member]      
Transfers of Financial Assets [Line Items]      
Retained interests: principal and interest received 1.1 1.2 $ 1.2
Unconsolidated VIEs [Member] | Single Class MBS, REMIC & Megas [Member]      
Transfers of Financial Assets [Line Items]      
Retained interests: unpaid principal balance 3.9 4.4  
Retained interest: fair value $ 4.7 $ 5.8