Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Investments [Abstract]
Cash Equivalents		$ 1.0		$ 3.8
Recurring Fair Value Measures [Member]
Investments [Abstract]
Available-for-sale – Equity Securities	[1]	12.2		10.2
Available-for-sale – Corporate and Governmental Debt Securities	[1]	8.0		8.9
Cash Equivalents	[1]	1.0		3.8
Total Fair Value of Assets		21.2		22.9
Liabilities [Abstract]
Deferred Compensation	[2]	19.8		18.2
U.S. Water Services Contingent Consideration		3.8	[3]	5.4	[2]
Total Fair Value of Liabilities		23.6		23.6
Total Net Fair Value of Assets (Liabilities)		(2.4)		(0.7)
Activity in Level 3 [Roll Forward]
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]	12.2		10.2
Available-for-sale – Corporate and Governmental Debt Securities	[1]	0.0		0.0
Cash Equivalents	[1]	1.0		3.8
Total Fair Value of Assets		13.2		14.0
Liabilities [Abstract]
Deferred Compensation	[2]	0.0		0.0
U.S. Water Services Contingent Consideration		0.0	[3]	0.0	[2]
Total Fair Value of Liabilities		0.0		0.0
Total Net Fair Value of Assets (Liabilities)		13.2		14.0
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]	8.0		8.9
Cash Equivalents	[1]	0.0		0.0
Total Fair Value of Assets		8.0		8.9
Liabilities [Abstract]
Deferred Compensation	[2]	19.8		18.2
U.S. Water Services Contingent Consideration		0.0	[3]	0.0	[2]
Total Fair Value of Liabilities		19.8		18.2
Total Net Fair Value of Assets (Liabilities)		(11.8)		(9.3)
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		0.0
Cash Equivalents	[1]	0.0		0.0
Total Fair Value of Assets		0.0		0.0
Liabilities [Abstract]
Deferred Compensation	[2]	0.0		0.0
U.S. Water Services Contingent Consideration		3.8	[3]	5.4	[2]
Total Fair Value of Liabilities		3.8		5.4
Total Net Fair Value of Assets (Liabilities)		(3.8)		(5.4)
Activity in Level 3 [Roll Forward]
Beginning Balance		5.4		25.0
Ending Balance		3.8		5.4
Recurring Fair Value Measures [Member] | Level 3 [Member] | Accretion [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3	[4]	(0.4)		(0.8)
Recurring Fair Value Measures [Member] | Level 3 [Member] | Payments [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3	[5]			19.7
Recurring Fair Value Measures [Member] | Level 3 [Member] | Changes in Cash Flow Projections [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3		$ 2.0		$ 0.7

[1] Included in Other Investments on the Consolidated Balance Sheet.

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

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

[4] Included in Interest Expense on the Consolidated Statement of Income.

[5] Payments reflect the impact of a modification to the shareholder agreement in the first quarter of 2017 which provided participants a one-time election to sell shares at a determined price. Participants representing approximately half of the outstanding contingent consideration shares made the election, and were paid in 2017.