Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding (loss) gain on investment securities arising during the period, net of tax	$ (336.1)	$ 201.8	$ (182.2)
Reclassification adjustment for net realized gain on investment securities, net of tax	(48.2)	(83.2)	(112.5)
Total reclassification adjustment on investments	(384.3)	118.6	(294.7)
Change in non-credit component of other-than-temporary impairment losses on investments	(5.6)	(3.9)	1.7
Cash flow hedges, holding (loss) gain, net of tax	(45.2)	(3.6)	3.0
Change in net periodic pension and postretirement benefits	(26.0)	(118.1)	172.7
Foreign currency translation adjustment, net of tax	(3.4)	(4.3)	1.4
Net loss recognized in other comprehensive income, net of tax	$ (464.5)	$ (11.3)	$ (115.9)