Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Pension [Member]
Contributions by employer			$ 9.8	$ 7.5
Service cost	$ 1.1	$ 1.0	2.2	1.9
Interest cost	2.0	2.9	4.0	5.9
Expected return on assets	(3.7)	(4.7)	(7.4)	(9.4)
Prior service cost	0.2	0.2	0.4	0.5
Actuarial loss / (gain)	2.3	1.6	4.6	3.1
Net periodic benefit cost	1.9	1.0	3.8	2.0
Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	8.5		8.5		$ 9.0
Subsidiaries [Member] | Pension [Member]
Contributions by employer			9.8	7.5
Service cost	1.1	1.0	2.2	1.9
Interest cost	2.0	2.9	4.0	5.9
Expected return on assets	(3.7)	(4.7)	(7.4)	(9.4)
Prior service cost	0.3	0.4	0.6	0.7
Actuarial loss / (gain)	2.8	2.1	5.6	4.3
Net periodic benefit cost	2.5	$ 1.7	5.0	$ 3.4
Subsidiaries [Member] | Postretirement [Member]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 8.5		$ 8.5		$ 9.0