Accumulated Other Comprehensive Loss (Reconciliation Of The Components Of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	$ 939.7	$ 748.6
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(104.9)	(180.9)
Investments, net pretax unrealized gains, excluding non-credit component of other-than-temporary impairments	834.8	567.7
Investments, deferred tax liability	(301.1)	(206.5)
Net unrealized gains on investments excluding non-credit component of other-than-temporary impairments	533.7	361.2
Non-credit components of other-than-temporary impairments on investments, Gross unrealized losses	(0.3)	(7.2)
Non-credit components of other-than-temporary impairments on investments, Deferred tax asset	0.1	2.6
Net unrealized non-credit component of other-than-temporary impairments on investments	(0.2)	(4.6)
Cash flow hedges, Gross unrealized losses	(358.6)	(259.1)
Cash flow hedges, Deferred tax asset	125.6	90.7
Net unrealized losses on cash flow hedges	(233.0)	(168.4)
Defined benefit pension plans, Deferred net actuarial loss	(624.7)	(655.8)
Defined benefit pension plans, Deferred prior service credits	(1.0)	(0.5)
Defined benefit pension plans, Deferred tax asset	244.1	257.2
Net unrecognized periodic benefit costs for defined benefit pension plans	(381.6)	(399.1)
Postretirement benefit plans, deferred net actuarial loss	(77.5)	(146.6)
Postretirement benefit plans, deferred prior service credit	46.1	59.7
Postretirement benefit plans, Deferred tax asset	12.3	34.0
Net unrecognized periodic benefit costs for postretirement benefit plans	(19.1)	(52.9)
Foreign currency translation adjustments, Gross unrealized losses	(2.1)	(6.3)
Foreign currency translation adjustments, Deferred tax asset	0.8	2.2
Net unrealized losses on foreign currency translation adjustment	(1.3)	(4.1)
Accumulated other comprehensive loss	$ (101.5)	$ (267.9)