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Investment Securities Summary of Cumulative Other-than-Temporary Impairment Charges Recognized in Earnings for Pooled Trust Preferred Securities Held (Details) - Pooled trust preferred securities - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]    
Balance of cumulative credit losses on debt securities, beginning of year $ (11.5) $ (11.5)
Balance of cumulative credit losses on debt securities, end of year $ (11.5) $ (11.5)