Pension and Other Postretirement Benefits (Schedule Of Reconciliation Of The Plans Funded Status And Amounts Reflected) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Noncurrent liabilities	$ (303.5)	$ (314.3)
Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	2,342.3
Fair value of plan assets at end of year	2,404.1	2,342.3
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	2,130.5	1,981.3
Service cost	32.0	29.2	$ 31.3
Interest cost	51.6	72.3	67.1
Plan participants' contributions	0.0	0.0
Plan amendments	0.0	0.0
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	140.1	204.3
Defined Benefit Plan, Benefit Obligation, Benefits Paid	174.5	156.6
Benefits paid	(174.6)	(156.5)
Spinoff to Eversource	121.4	0.0
Estimated benefits paid by incurred subsidy	0.0	0.0
Projected benefit obligation at end of year	2,058.4	2,130.5	1,981.3
Actual return on plan assets	329.9	366.8
Employer contributions	2.9	2.9
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	0.0	0.0
Defined Benefit Plan, Funded (Unfunded) Status of Plan	59.3	(49.6)
Noncurrent assets	91.4	8.2
Current liabilities	(2.9)	(3.0)
Noncurrent liabilities	(29.2)	(54.8)
Net amount recognized at end of year	59.3	(49.6)
Unrecognized prior service cost	0.3	3.0
Unrecognized actuarial loss	497.2	652.8
Defined Benefit Plan Amounts Recognized In Other Comprehensive Income Or Regulatory Asset Or Liability	497.5	655.8
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	2,080.9	1,867.7
Fair value of plan assets at end of year	2,117.7	2,080.9	1,867.7
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	576.5	492.5
Service cost	6.6	5.1	5.0
Interest cost	15.4	19.2	17.6
Plan participants' contributions	4.1	4.8
Plan amendments	0.0	5.1
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	24.8	88.8
Defined Benefit Plan, Benefit Obligation, Benefits Paid	37.0	39.5
Benefits paid	(37.0)	(39.5)
Spinoff to Eversource	0.0	0.0
Estimated benefits paid by incurred subsidy	0.4	0.5
Projected benefit obligation at end of year	590.8	576.5	492.5
Actual return on plan assets	36.3	56.9
Employer contributions	21.6	23.0
Defined Benefit Plan, Plan Assets, Contributions by Plan Participant	4.1	4.7
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(304.4)	(315.1)
Noncurrent assets	0.0	0.0
Current liabilities	(0.9)	(0.8)
Net amount recognized at end of year	(304.4)	(315.1)
Unrecognized prior service cost	(10.1)	(10.7)
Unrecognized actuarial loss	116.4	118.4
Defined Benefit Plan Amounts Recognized In Other Comprehensive Income Or Regulatory Asset Or Liability	106.3	107.7
Other Postretirement Benefits | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	261.4	216.3
Fair value of plan assets at end of year	$ 286.4	$ 261.4	$ 216.3