Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019		Dec. 31, 2018
Investments [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value		$ 2.4
Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value		6.1
Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value		0.7
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]	10.6		$ 12.2
Available-for-sale – Corporate and Governmental Debt Securities (b)	[1]	9.6	[2]	8.0
Cash Equivalents	[1]	1.2		1.0
Total Fair Value of Assets		21.4		21.2
Liabilities [Abstract]
Deferred Compensation	[3]	21.6		19.8
U.S. Water Services Contingent Consideration	[4]			3.8
Total Fair Value of Liabilities		21.6		23.6
Total Net Fair Value of Assets (Liabilities)		(0.2)		(2.4)
Recurring Fair Value Measures [Member] | Level 1 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	10.6		12.2
Available-for-sale – Corporate and Governmental Debt Securities (b)	[1]	0.0	[2]	0.0
Cash Equivalents	[1]	1.2		1.0
Total Fair Value of Assets		11.8		13.2
Liabilities [Abstract]
Deferred Compensation	[3]	0.0		0.0
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)		11.8		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 (b)	[1]	9.6	[2]	8.0
Cash Equivalents	[1]	0.0		0.0
Total Fair Value of Assets		9.6		8.0
Liabilities [Abstract]
Deferred Compensation	[3]	21.6		19.8
U.S. Water Services Contingent Consideration	[4]			0.0
Total Fair Value of Liabilities		21.6		19.8
Total Net Fair Value of Assets (Liabilities)		(12.0)		(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 (b)	[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	[3]	0.0		0.0
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)
Recurring Fair Value Measures [Member] | Level 3 [Member] | Payments [Member]
Liabilities [Abstract]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease)		$ 3.8

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

[2] As of September 30, 2019, the aggregate amount of available-for-sale corporate and governmental debt securities maturing in one year or less was $2.4 million, in one year to less than three years was $6.1 million, in three years to less than five years was $0.7 million 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 Current Liabilities on the Consolidated Balance Sheet.