Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Net income	$ (5.2)	$ 1.2	$ 0.2	$ 15.4	$ (4.0)	$ 15.6
Derivative activity:
Reclassification of earnings, net of income tax	(0.2)		(0.3)		(0.4)	(0.4)
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.1		0.3		0.1	0.5
Other comprehensive income / (loss)	(0.1)	(0.2)	0.0	0.1	(0.3)	0.1
Net comprehensive income / (loss)	(5.3)		0.2		(4.3)	15.7
Subsidiaries [Member]
Net income	(0.4)		7.3	16.9	9.2	24.2
Pension and postretirement activity:
Reclassification to earnings, net of income tax	0.1		0.7		0.2	1.4
Other comprehensive income / (loss)	0.1	0.1	0.7	$ 0.7	0.2	1.4
Net comprehensive income / (loss)	(0.3)		8.0		9.4	25.6
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)		$ (0.7)		$ (0.2)	$ (1.4)