Benefit Plans (Net Periodic Benefit Cost (Income)) (Details) - Pension [Member] - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Contributions by employer	$ 7.5	$ 7.5
Service cost	0.6	0.7
Interest cost	3.7	4.0
Expected return on assets	(3.7)	(4.4)
Prior service cost	0.3	0.2
Actuarial loss / (gain)	0.5	0.2
Net periodic benefit cost	1.4	0.7
Subsidiaries [Member]
Contributions by employer	7.5	7.5
Service cost	0.6	0.7
Interest cost	3.7	4.0
Expected return on assets	(3.7)	(4.4)
Prior service cost	0.2	0.3
Actuarial loss / (gain)	1.0	0.3
Net periodic benefit cost	$ 1.8	$ 0.9