Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Net income	$ 38.6	$ (5.2)	$ 1.2	$ (19.7)	$ 0.2	$ 15.4	$ 34.6	$ (4.1)
Derivative activity:
Reclassification of earnings, net of income tax	(0.2)			(0.1)			(0.6)	(0.5)
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.0			0.2			0.1	0.7
Other comprehensive income / (loss)	(0.2)	(0.1)	(0.2)	0.1	0.0	0.1	(0.5)	0.2
NET COMPREHENSIVE INCOME / (LOSS)	38.4			(19.6)			34.1	(3.9)
Subsidiaries [Member]
Net income	43.6			(10.1)	7.3	16.9	52.8	14.1
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.1			0.6			0.3	2.0
Other comprehensive income / (loss)	0.1	0.1	0.1	0.6	$ 0.7	$ 0.7	0.3	2.0
NET COMPREHENSIVE INCOME / (LOSS)	43.7			(9.5)			53.1	16.1
Retained Earnings [Member] | Subsidiaries [Member]
Net income		$ (0.4)	$ 9.6
Change in unfunded pension obligation [Member] | Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Net income	$ (0.1)			$ (0.6)			$ (0.3)	$ (2.0)