Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Fair value investments	$ 2,509.4	$ 2,547.3	$ 2,406.9	$ 1,543.5