Accumulated Other Comprehensive Income (Loss) (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain on investment securities arising during the period, net of tax expense of ($67.5) and ($2.4), respectively	$ 120.2	$ 29.7
Reclassification adjustment for net realized gain on investment securities, net of tax expense of ($10.8) and ($2.6), respectively	20.1	4.8
Total reclassification adjustment on investments	140.3	34.5
Non-credit component of other-than-temporary impairments on investments, net of tax expense of ($0.3) and ($1.2), respectively	0.5	2.2
Cash flow hedges, holding gain, net of tax expense of ($0.4) and ($0.4), respectively	0.7	0.7
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax expense of ($1.9) and ($4.4), respectively	2.9	6.8
Foreign currency translation adjustment, net of tax benefit of $0.0 and $0.5, respectively	(0.2)	(0.8)
Net gain (loss) recognized in other comprehensive income (loss), net of tax expense (benefit)	$ 144.2	$ 43.4