Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Contributions by employer			$ 7.5	$ 7.5
Pension [Member]
Service cost	$ 0.9	$ 0.9	2.8	2.7
Interest cost	2.9	3.7	8.8	11.2
Expected return on assets	(4.6)	(5.0)	(14.0)	(15.0)
Prior service cost	0.3	0.3	0.8	0.9
Actuarial loss / (gain)	1.5	1.1	4.6	3.2
Net periodic benefit cost	1.0	1.0	3.0	3.0
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	9.1		9.1		$ 9.6
Subsidiaries [Member] | Pension [Member]
Contributions by employer			7.5	7.5
Service cost	0.9	0.9	2.8	2.7
Interest cost	2.9	3.7	8.8	11.2
Expected return on assets	(4.6)	(5.0)	(14.0)	(15.0)
Prior service cost	0.3	0.4	1.0	1.3
Actuarial loss / (gain)	2.2	1.8	6.5	5.3
Net periodic benefit cost	1.7	$ 1.8	5.1	$ 5.5
Subsidiaries [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 9.1		$ 9.1		$ 9.6