Short-Term Debt and Lines of Credit - Credit Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) extension	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Available capacity under existing agreements	$ 383.5
Number of extensions available on a credit facility | extension	2
Length of credit facility extension	1 year
Credit facility maturing October 2022 [Member]
Line of Credit Facility [Line Items]
Short-term credit capacity	$ 500.0
Letters of Credit
Line of Credit Facility [Line Items]
Letters of credit issued inside credit facilities	1.0
Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 115.5	$ 134.9