Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Liabilities [Abstract]
Activity in Level 3		$ (3.8)
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value		2.1
Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value		7.2
Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value		0.0
Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value		0.4
Recurring Fair Value Measures [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	11.1		$ 12.2
Available-for-sale – Corporate and Governmental Debt Securities	[1]	9.7	[2]	8.0
Cash Equivalents	[1]	0.9		1.0
Total Fair Value of Assets		21.7		21.2
Liabilities [Abstract]
Deferred Compensation		21.2	[3]	19.8	[4]
U.S. Water Services Contingent Consideration	[4]			3.8
Total Fair Value of Liabilities		21.2		23.6
Total Net Fair Value of Assets (Liabilities)		0.5		(2.4)
Fair Value Hierarchy Transfers, All Levels		0.0		0.0
Recurring Fair Value Measures [Member] | Level 1 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	11.1		12.2
Available-for-sale – Corporate and Governmental Debt Securities	[1]	0.0	[2]	0.0
Cash Equivalents	[1]	0.9		1.0
Total Fair Value of Assets		12.0		13.2
Liabilities [Abstract]
Deferred Compensation		0.0	[3]	0.0	[4]
U.S. Water Services Contingent Consideration	[4]			0.0
Total Fair Value of Liabilities		0.0		0.0
Total Net Fair Value of Assets (Liabilities)		12.0		13.2
Recurring Fair Value Measures [Member] | Level 2 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	0.0		0.0
Available-for-sale – Corporate and Governmental Debt Securities	[1]	9.7	[2]	8.0
Cash Equivalents	[1]	0.0		0.0
Total Fair Value of Assets		9.7		8.0
Liabilities [Abstract]
Deferred Compensation		21.2	[3]	19.8	[4]
U.S. Water Services Contingent Consideration	[4]			0.0
Total Fair Value of Liabilities		21.2		19.8
Total Net Fair Value of Assets (Liabilities)		(11.5)		(11.8)
Recurring Fair Value Measures [Member] | Level 3 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	0.0		0.0
Available-for-sale – Corporate and Governmental Debt Securities	[1]	0.0	[2]	0.0
Cash Equivalents	[1]	0.0		0.0
Total Fair Value of Assets		0.0		0.0
Liabilities [Abstract]
Deferred Compensation		0.0	[3]	0.0	[4]
U.S. Water Services Contingent Consideration	[4]			3.8
Total Fair Value of Liabilities		0.0		3.8
Total Net Fair Value of Assets (Liabilities)		$ 0.0		$ (3.8)

[1] Included in Other Non-Current Assets on the Consolidated Balance Sheet.

[2] As of December 31, 2019, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $2.1 million, in one year to less than three years was $7.2 million, in three years to less than five years was zero and in five or more years was $0.4 million.

[3] Included in Other Non-Current Liabilities on the Consolidated Balance Sheet.

[4] Included in Other Non-Current Liabilities on the Consolidated Balance Sheet.