Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain (loss) on investment securities arising during the period, net of tax (expense) benefit of ($18.3), $223.6 and $5.4, respectively	$ 35.4	$ (407.2)	$ (3.2)
Reclassification adjustment for net realized gain on investment securities, net of tax expense of ($44.8), ($60.6) and ($104.0), respectively	83.2	112.5	193.1
Total reclassification adjustment on investments	118.6	(294.7)	189.9
Non-credit component of OTTI on investments, net of tax benefit (expense) of $2.1, ($0.9) and ($2.4), respectively	(3.9)	1.7	4.5
Holding (loss) gain, net of tax benefit (expense) of $1.9, ($1.6) and ($0.1), respectively	(3.6)	3.0	0.1
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax benefit (expense) of $75.2, ($106.8) and ($7.1), respectively	(118.1)	172.7	(10.9)
Foreign currency translation adjustment, net of tax benefit (expense) of $2.2, ($0.6) and ($0.3), respectively	(4.3)	1.4	0.6
Net (loss) gain recognized in other comprehensive (loss) income, net of tax benefit (expense) of $18.3, $53.1 and ($108.5), respectively	$ (11.3)	$ (115.9)	$ 184.2