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Fair Value - Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Recurring Fair Value Measure, Unobservable Inputs, Period Increase (Decrease)	$ 3.8
Equity Investments	197.6	$ 161.1
ATC [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Investments	141.6	128.1	$ 118.7
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] \| Corporate and Other [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Impairment of Long-Lived Assets Held-for-use	0.0
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] \| ATC [Member] \| Regulated Operations [Member]
Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Line Items]
Equity Method Investment, Indicators of Impairment	$ 0.0	$ 0.0