Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain (loss) on investment securities arising during the period, net of tax	$ 94.2	$ 147.1
Reclassification adjustment for net realized gain (loss) on investment securities, net of tax	(14.0)	25.2
Total reclassification adjustment on investments	80.2	172.3
Non-credit component of other-than-temporary impairments on investments, net of tax	3.6	(1.7)
Cash flow hedges, holding gain (loss), net of tax	17.0	(265.5)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax	3.9	3.8
Foreign currency translation adjustment, net of tax	1.4	1.3
Net gain (loss) recognized in other comprehensive income, net of tax	$ 106.1	$ (89.8)