Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Investments [Abstract]
Cash Equivalents		$ 3.8		$ 1.3
Liabilities [Abstract]
U.S. Water Services Contingent Consideration	[1]	5.4		25.0
Recurring Fair Value Measures [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		10.2	[2]	7.1	[3]
Available-for-sale – Corporate and Governmental Debt Securities		8.9	[2]	11.7	[3]
Cash Equivalents		3.8	[2]	1.3	[3]
Total Fair Value of Assets		22.9		20.1
Liabilities [Abstract]
Deferred Compensation		18.2	[4]	16.0	[5]
U.S. Water Services Contingent Consideration		5.4	[4]	25.0	[5]
Total Fair Value of Liabilities		23.6		41.0
Total Net Fair Value of Assets (Liabilities)		(0.7)		(20.9)
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		10.2	[2]	7.1	[3]
Available-for-sale – Corporate and Governmental Debt Securities		0.0	[2]	0.0	[3]
Cash Equivalents		3.8	[2]	1.3	[3]
Total Fair Value of Assets		14.0		8.4
Liabilities [Abstract]
Deferred Compensation		0.0	[4]	0.0	[5]
U.S. Water Services Contingent Consideration		0.0	[4]	0.0	[5]
Total Fair Value of Liabilities		0.0		0.0
Total Net Fair Value of Assets (Liabilities)		14.0		8.4
Recurring Fair Value Measures [Member] | Level 2 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		0.0	[2]	0.0	[3]
Available-for-sale – Corporate and Governmental Debt Securities		8.9	[2]	11.7	[3]
Cash Equivalents		0.0	[2]	0.0	[3]
Total Fair Value of Assets		8.9		11.7
Liabilities [Abstract]
Deferred Compensation		18.2	[4]	16.0	[5]
U.S. Water Services Contingent Consideration		0.0	[4]	0.0	[5]
Total Fair Value of Liabilities		18.2		16.0
Total Net Fair Value of Assets (Liabilities)		(9.3)		(4.3)
Recurring Fair Value Measures [Member] | Level 3 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		0.0	[2]	0.0	[3]
Available-for-sale – Corporate and Governmental Debt Securities		0.0	[2]	0.0	[3]
Cash Equivalents		0.0	[2]	0.0	[3]
Total Fair Value of Assets		0.0		0.0
Liabilities [Abstract]
Deferred Compensation		0.0	[4]	0.0	[5]
U.S. Water Services Contingent Consideration		5.4	[4]	25.0	[5]
Total Fair Value of Liabilities		5.4		25.0
Total Net Fair Value of Assets (Liabilities)		(5.4)		(25.0)
Activity in Level 3 [Roll Forward]
Beginning Balance		25.0		36.6
Ending Balance		5.4		25.0
Recurring Fair Value Measures [Member] | Level 3 [Member] | Accretion [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3	[6]	(0.8)		(2.8)
Recurring Fair Value Measures [Member] | Level 3 [Member] | Payments [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3		19.7	[7]	0.8
Recurring Fair Value Measures [Member] | Level 3 [Member] | Changes in Cash Flow Projections [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3		$ 0.7	[7]	$ 13.6	[8]

[1] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 6. Acquisitions and Note 9. Fair Value.)

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

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

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

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

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

[7] Payments and changes in cash flow projections 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 the first half of 2017.

[8] During the fourth quarter of 2016, management assessed earnings estimates used in calculating the fair value of the U.S. Water Services contingent consideration liability and determined an adjustment was necessary to the liability’s carrying amount based on its assessment. As a result, we recorded a reduction of $13.6 million to the liability’s carrying amount which resulted in an after-tax gain of the same amount presented within Operating Expenses – Other in the Consolidated Statement of Income.