Accumulated Other Comprehensive Loss (Reconciliation Of The Components Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Document Fiscal Year Focus	2017
Investments, gross unrealized gains excluding non-credit component of other-than-temporary impairments	$ 940.9	$ 1,068.9
Investments, gross unrealized losses excluding non-credit component of other-than-temporary impairments	(80.1)	(74.2)
Investments, net pretax unrealized gains excluding non-credit component of other-than-temporary impairments	860.8	994.7
Deferred tax liability	(309.9)	(364.0)
Net unrealized gains on investments excluding non-credit component of other-than-temporary impairments	550.9	630.7
Non-credit component of other than temporary impairments on investments, unrealized losses	(0.2)	(12.1)
Non-credit component of other than temporary impairments on investments, deferred tax asset	0.1	4.3
Net unrealized non-credit component of other than temporary impairments on investments	(0.1)	(7.8)
Cash flow hedges, gross unrealized losses	(363.2)	(852.0)
Cash flow hedges, deferred tax asset	127.1	298.2
Net unrealized losses on cash flow hedges	(236.1)	(553.8)
Defined benefit pension plans, deferred net actuarial loss	(632.9)	(615.8)
Defined benefit pension plans, deferred prior service credits	(0.8)	(0.3)
Defined benefit pension plans, deferred tax asset	248.4	243.0
Defined Benefit Plan Accumulated Other Comprehensive Income Adjustments Net Of Tax	(385.3)	(373.1)
Postretirement benefit plans, deferred net actuarial loss	(138.0)	(153.4)
Deferred Prior Service Cost (Credit) Postretirement Benefit Plans	49.5	63.2
Defined benefit pension plans, deferred tax asset	34.6	35.5
Net unrecognized periodic benefit costs for postretirement benefit plans	(53.9)	(54.7)
Foreign currency translation adjustments, gross unrealized losses	(2.4)	(7.9)
Foreign currency translation adjustments, deferred tax asset	0.9	2.7
Net unrealized losses on foreign currency translation adjustments	(1.5)	(5.2)
Accumulated other comprehensive loss	$ (126.0)	$ (363.9)	$ (267.9)