Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income Reclassification Adjustments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Parantheticals) [Abstract]
Net holding gain on investment securities arising during the period, tax expense	$ (48.5)	$ (36.7)	$ (161.2)	$ (275.8)
Reclassification adjustment for net realized gain on investment securities, tax expense	42.6	31.3	57.8	36.0
Non-credit component of other than temporary impairments on investments, tax (expense) benefit	(0.2)	(4.9)	(2.7)	(1.0)
Cash flow hedges, holding loss, tax benefit	2.5	9.2	36.4	254.5
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, tax expense	(3.0)	(1.7)	(8.2)	(7.0)
Foreign currency translation adjustment, tax (expense) benefit	(0.2)	(0.3)	(1.3)	(0.5)
Net gain (loss) recognized in other comprehensive income (loss), tax (expense) benefit	$ (6.8)	$ (3.1)	$ (79.2)	$ 6.2