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Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]        
Fair value investments $ 2,275.4 $ 3,208.3 $ 2,633.8 $ 2,547.3