Accumulated Other Comprehensive Income (Loss) Accumulated Other Comprehensive Income Reclassification Adjustments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Parantheticals) [Abstract]
Net holding gain (loss) on investment securities arising during the period, tax (expense) benefit	$ (46.3)	$ (85.4)
Reclassification adjustment for net realized (gain) loss on investment securities, tax expense (benefit)	7.5	(13.6)
Non-credit component of other than temporary impairments on investments, tax benefit (expense)	(2.1)	1.2
Cash flow hedges, holding gain (loss), tax (expense) benefit	(2.7)	142.9
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, tax expense	(2.3)	(2.4)
Foreign currency translation adjustment, tax benefit (expense)	(0.7)	(0.7)
Net gain (loss) recognized in other comprehensive income (loss), tax benefit (expense)	$ (46.6)	$ 42.0