Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain (loss) on investment securities arising during the period, net of tax	$ 243.4	$ (195.4)	$ 390.9	$ (113.9)
Reclassification adjustment for net realized gain on investment securities, net of tax	(33.6)	(45.8)	(8.8)	(66.9)
Total reclassification adjustment on investments	209.8	(241.2)	382.1	(180.8)
Non-credit component of other-than-temporary impairments on investments, net of tax	(5.4)	(2.8)	(7.1)	0.7
Cash flow hedges, holding (loss) gain, net of tax	(190.0)	0.9	(455.5)	1.8
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax	3.8	4.9	7.6	9.6
Foreign currency translation adjustment, net of tax	(0.7)	0.7	0.6	(2.8)
Net gain (loss) recognized in other comprehensive income, net of tax	$ 17.5	$ (237.5)	$ (72.3)	$ (171.5)