Supplemental Financial Information (Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		$ (3.6)				$ (3.6)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.0
Other comprehensive income / (loss)	$ 0.1	(0.3)	$ (1.0)		$ (0.4)	0.0	$ (2.2)
Balance, end of period	(3.6)					(3.6)
Interest and Debt Expense	(17.1)		(18.3)			(55.4)	(63.7)
Income Tax Expense (Benefit)	10.4		9.1			7.7	11.8
Net income	(8.2)	1.9	35.4		42.1	(2.8)	61.4
Income tax expense / (benefit) from discontinued operations	0.3		(0.2)			0.8	6.3
Nonoperating Income (Expense)	(47.8)		(17.9)			(86.2)	(105.4)
Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		(36.9)				(36.9)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(2.1)
Other comprehensive income / (loss)	0.9		1.6	$ (0.9)	$ 0.4	2.2	1.1
Balance, end of period	(34.7)					(34.7)
Interest and Debt Expense	(5.9)		(6.1)			(18.5)	(19.9)
Income Tax Expense (Benefit)	(3.5)		7.2			(2.4)	(5.1)
Net income	17.8		44.9			47.3	103.7
Nonoperating Income (Expense)	(5.7)		(6.6)			(19.9)	(19.8)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		14.5				14.5
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.8
Other comprehensive income / (loss)						(0.8)
Balance, end of period	13.7					13.7
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		(0.4)				(0.4)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.3
Other comprehensive income / (loss)						(0.2)
Balance, end of period	(0.6)					(0.6)
Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		(18.1)				(18.1)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(0.8)
Other comprehensive income / (loss)						0.8
Balance, end of period	(17.3)					(17.3)
Change in unfunded pension obligation [Member] | Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period		$ (36.5)				(36.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(2.4)
Other comprehensive income / (loss)						2.4
Balance, end of period	(34.1)					(34.1)
Reclassification out of Accumulated Other Comprehensive Income [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net income	0.0		(0.8)			0.0	(1.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net income	0.7		1.7			2.1	2.0
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Interest and Debt Expense	(0.3)		(0.3)			(0.9)	(0.9)
Income Tax Expense (Benefit)	0.0		(0.1)			0.1	0.0
Net income	(0.3)		(0.4)			(0.8)	(0.9)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Interest and Debt Expense	0.0		0.0			(0.2)	(0.1)
Income Tax Expense (Benefit)	(0.1)		0.0			(0.1)	0.0
Net income	(0.1)		0.0			(0.3)	(0.1)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	0.0		(0.1)			(0.1)	(0.1)
Net income	0.3		0.0			0.8	0.1
Nonoperating Income (Expense)	0.3		0.1			0.9	0.2
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member] | Subsidiaries [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income Tax Expense (Benefit)	(0.2)		0.9			(0.6)	(0.5)
Net income	0.8		1.7			2.4	2.1
Nonoperating Income (Expense)	1.0		0.8			3.0	2.6
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Income tax expense / (benefit) from discontinued operations	$ 0.0		$ (0.4)			$ 0.0	$ (0.4)