Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 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	$ 280.1	$ 186.0	$ (336.1)
Reclassification adjustment for net realized gain on investment securities, net of tax	(107.6)	(68.1)	(48.2)
Total reclassification adjustments on investments	172.5	117.9	(384.3)
Non-credit component of other-than-temporary impairments on investments, net of tax	4.4	5.4	(5.6)
Cash flow hedges, holding (loss) gain, net of tax	(64.6)	(87.3)	(45.2)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, net of tax	51.3	(13.4)	(26.0)
Foreign currency translation adjustment, net of tax	2.8	2.1	(3.4)
Net gain (loss) recognized in other comprehensive loss, net of tax	$ 166.4	$ 24.7	$ (464.5)