Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net holding gain (loss) on investment securities arising during the period, net of tax	$ 186.0	$ (336.1)	$ 201.8
Reclassification adjustment for net realized gain on investment securities, net of tax	(68.1)	(48.2)	(83.2)
Total reclassification adjustments on investments	117.9	(384.3)	118.6
Non-credit component of other-than-temporary impairments on investments, net of tax	5.4	(5.6)	(3.9)
Cash flow hedges, holding (loss) gain, net of tax	(87.3)	(45.2)	(3.6)
Change in net periodic pension and postretirement costs	(13.4)	(26.0)	(118.1)
Foreign currency translation adjustment, net of tax	2.1	(3.4)	(4.3)
Net gain (loss) recognized in other comprehensive income/loss, net of tax	$ 24.7	$ (464.5)	$ (11.3)