Short-Term Borrowings (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0.0	$ 0.0	$ 42.0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	100.0	100.0	100.0	$ 100.0
Amount outstanding at period end	0.0	0.0	0.0	0.0
Average amount outstanding	1.0	8.0	29.0	13.0
Maximum amount outstanding	$ 12.0	$ 100.0	$ 100.0	$ 100.0
Weighted average interest rate, computed on a daily basis (percentage)	1.63%	2.42%	1.96%	1.12%
Commercial Paper
Short-term Debt [Line Items]
Borrowing limit	$ 500.0	$ 500.0	$ 400.0	$ 400.0
Amount outstanding at period end	0.0	0.0	42.0	0.0
Average amount outstanding	0.0	72.0	30.0	69.0
Maximum amount outstanding	$ 0.0	$ 316.0	$ 144.0	$ 176.0
Weighted average interest rate, computed on a daily basis (percentage)		2.68%	2.27%	1.13%
Weighted average interest rate at period end (percentage)			2.80%