Accumulated Other Comprehensive Income (Reconciliation Of The Components Of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Investments, gross unrealized gains excluding non-credit component of other-than-temporary impairments	$ 811.0	$ 908.0
Investments, gross unrealized losses excluding non-credit component of other-than-temporary impairments	(124.3)	(269.2)
Investments, net pretax unrealized gains excluding non-credit component of other-than-temporary impairments	686.7	638.8
Deferred tax liability	(245.3)	(223.2)
Net unrealized gains on investments excluding non-credit component of other-than-temporary impairments	441.4	415.6
Non-credit component of other than temporary impairments on investments, unrealized losses	(1.5)	(18.3)
Non-credit component of other than temporary impairments on investments, deferred tax asset	0.5	6.6
Net unrealized non-credit component of other than temporary impairments on investments	(1.0)	(11.7)
Cash flow hedges, gross unrealized losses	(239.4)	(533.2)
Cash flow hedges, deferred tax asset	88.0	186.6
Net unrealized losses on cash flow hedges	(151.4)	(346.6)
Defined benefit pension plans, deferred net actuarial loss	(648.9)	(628.3)
Defined benefit pension plans, deferred prior service credits	(0.7)	(0.1)
Defined benefit pension plans, deferred tax asset	254.8	247.8
Net unrecognized periodic benefit costs for defined benefit pension plans	(394.8)	(380.6)
Postretirement benefit plans, deferred net actuarial loss	(143.7)	(159.6)
Postretirement benefit plans, deferred prior service credits	56.3	70.1
Defined benefit pension plans, deferred tax asset	34.1	35.3
Net unrecognized periodic benefit costs for postretirement benefit plans	(53.3)	(54.2)
Foreign currency translation adjustments, gross unrealized losses	(4.2)	(7.5)
Foreign currency translation adjustments, deferred tax asset	1.5	2.6
Net unrealized losses on foreign currency translation adjustments	(2.7)	(4.9)
Accumulated other comprehensive loss	$ (161.8)	$ (382.4)	$ (267.9)