Accumulated Other Comprehensive Income (Other Comprehensive Income (Loss) Reclassification Adjustments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) Reclassification Adjustments (Parantheticals) [Abstract]
Net holding gain (loss) investment securities arising during the period, tax (expense) benefit	$ 180.4	$ (107.9)	$ 102.4
Reclassification adjustment for net realized gain on investment securities, tax expense	25.9	44.8	60.6
Non-credit component of OTTI on investments, tax benefit (expense)	3.0	2.1	(0.9)
Cash flow hedges, holding (loss) gain, tax benefit (expense)	24.4	1.9	(1.6)
Net change in unrecognized periodic benefit costs for defined benefit pension and postretirement benefit plans, tax benefit (expense)	13.4	75.2	(106.8)
Foreign currency translation adjustment, tax benefit (expense)	1.8	2.2	(0.6)
Net (loss) gain realized in other comprehensive income, tax benefit (expense)	$ 197.1	$ (71.3)	$ (68.1)