Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Balance			$ 20,923.5
Net unrealized holding gains arising during the period	$ (100.2)	$ (66.6)	(91.0)	$ (10.7)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0.0	0.0	(0.3)	(3.2)
Net unrealized holding losses arising during the period	0.0	0.0	(4.7)	0.0
Foreign currency translation adjustment	(3.4)	(6.9)	(10.0)	(2.7)
Reclassification adjustment for gains included in investment income	(2.6)	0.0	3.4	0.0
Amortization of net actuarial (gains) losses included in pension expense	0.6	0.0	2.0	0.2
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	0.3	0.4	0.9	1.1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	0.0	5.0	(1.2)	6.5
Defined benefit plans, income tax benefit (expense)	0.2	0.0	0.6	(1.1)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax	(1.1)	(2.6)	(3.6)	(1.0)
Other Comprehensive Income (Loss), Net of Tax	(99.7)	(76.2)	(100.8)	(21.5)
Balance	21,376.8		21,376.8
Accumulated Other Comprehensive Income (Loss) [Member]
Balance			6.2	152.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax			(0.5)	0.2
Comprehensive Income Before Reclassification Adjustments			(106.0)	(17.7)
Comprehensive Income, Tax			5.7	(4.0)
Other Comprehensive Income (Loss), Net of Tax			(100.8)	(21.5)
Balance	(94.6)	130.5	(94.6)	130.5
Accumulated Translation Adjustment [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Balance			(2.2)	0.9
Reclassification adjustment for loss included in net income			0.0	0.0
Foreign currency translation adjustment			(10.0)	(2.7)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax			3.6	1.0
Other Comprehensive Income (Loss), Net of Tax			(6.4)	(1.7)
Balance	(8.6)	(0.8)	(8.6)	(0.8)
Investment Securities [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Balance			(22.9)	98.9
Net unrealized holding gains arising during the period			(91.0)	(10.7)
Reclassification adjustment for gains included in investment income			(3.4)	0.0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax			1.2	(6.5)
Other Comprehensive Income (Loss), Net of Tax			(93.2)	(17.2)
Balance	(116.1)	81.7	(116.1)	81.7
Derivative Financial Instruments, Assets [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Balance			62.6	65.0
Net unrealized holding losses arising during the period			(4.7)	(1.1)
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax			0.9	0.0
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax			1.5	0.4
Other Comprehensive Income (Loss), Net of Tax			(2.3)	(0.7)
Balance	60.3	64.3	60.3	64.3
Pension Plans, Defined Benefit [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Balance			(31.3)	(12.8)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax			(0.3)	(3.2)
Amortization of net actuarial (gains) losses included in pension expense			2.0	0.2
Defined benefit plans, income tax benefit (expense)			(0.6)	1.1
Other Comprehensive Income (Loss), Net of Tax			1.1	(1.9)
Balance	$ (30.2)	$ (14.7)	$ (30.2)	$ (14.7)