Accumulated Other Comprehensive Income (Loss) (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding (loss) gain on investment securities arising during the period, net of tax (benefit) expense	$ (15.0)	$ 135.3	$ (390.1)	$ 111.1
Reclassification adjustment for net realized gain on investment securities, net of tax expense	31.3	33.0	63.1	137.4
Total reclassification adjustment on investments	16.3	168.3	(327.0)	248.5
Non-credit component of other-than-temporary impairment losses on investments, net of tax (benefit) expense	(0.6)	0	1.6	4.4
Cash flow hedges, holding gain (loss), net of tax expense (benefit)	0.7	(1.0)	2.2	(0.6)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax expense	4.8	6.9	18.6	19.9
Foreign currency translation adjustment, net of tax expense (benefit)	1.2	0.5	0.7	(0.2)
Net gain (loss) recognized in other comprehensive income (loss), net of tax expense (benefit)	22.4	174.7	(303.9)	272.0
Net holding (loss) gain on investment securities arising during the period, tax (benefit) expense	(9.4)	73.4	(213.6)	59.1
Reclassification adjustment for net realized gains on investment securities, tax expense	16.9	17.8	34.0	74.0
Non-credit component of other-than-temporary impairment losses on investments, tax (benefit) expense	(0.3)	0	0.9	2.4
Cash flow hedges, holding gain (loss), tax expense (benefit)	0.4	(0.6)	1.2	(0.3)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, tax expense	3.2	4.7	12.1	13.5
Foreign currency translation adjustment, tax expense (benefit)	0.6	0.3	0.4	(0.1)
Net gain (loss) recognized in other comprehensive income (loss), tax expense (benefit)	$ 11.4	$ 95.6	$ (165.0)	$ 148.6