Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain on investment securities arising during the period, net of tax	$ 87.6	$ 63.4	$ 297.0	$ 454.3
Reclassification adjustment for net realized gain on investment securities, net of tax	(79.0)	(58.1)	(107.3)	(66.9)
Total reclassification adjustment on investments	8.6	5.3	189.7	387.4
Non-credit component of other-than-temporary impairments on investments, net of tax	0.0	9.3	4.5	2.2
Cash flow hedges, holding gain (loss), net of tax	(4.9)	(17.2)	(67.7)	(472.7)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax	4.7	3.2	12.8	10.8
Foreign currency translation adjustment, net of tax	0.5	0.4	2.6	1.0
Net gain (loss) recognized in other comprehensive income, net of tax	$ 8.9	$ 1.0	$ 141.9	$ (71.3)