Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Parantheticals) [Abstract]
Net holding gain (loss) on investment securities arising during the period, tax (expense) benefit	$ (152.6)	$ (118.9)	$ 180.4
Reclassification adjustment for net realized gain on investment securities, tax expense	58.0	36.6	25.9
Non-credit component of other-than-temporary impairments on investments, tax (expense) benefit	(2.8)	(2.8)	3.0
Cash flow hedges, holding loss, tax benefit	34.9	47.0	24.4
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, tax (expense) benefit	(34.8)	5.7	13.4
Foreign currency translation adjustment, tax (expense) benefit	(1.4)	(1.1)	1.8
Net gain (loss) realized in other comprehensive loss, tax (expense) benefit	$ (98.7)	$ (33.5)	$ 248.9