Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015
Investments [Abstract]
Cash Equivalents		$ 1.3		$ 2.0
Liabilities [Abstract]
U.S. Water Services Contingent Consideration	[1]	25.0		36.6
Recurring Fair Value Measures [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		7.1	[2]	7.6	[3]
Available-for-sale – Corporate and Governmental Debt Securities		11.7	[2]	10.9	[3]
Cash Equivalents		1.3	[2]	2.0	[3]
Total Fair Value of Assets		20.1		20.5
Liabilities [Abstract]
Deferred Compensation		16.0	[4]	16.1	[5]
U.S. Water Services Contingent Consideration		25.0	[4]	36.6	[5]
Total Fair Value of Liabilities		41.0		52.7
Total Net Fair Value of Assets (Liabilities)		(20.9)		(32.2)
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		7.1	[2]	7.6	[3]
Available-for-sale – Corporate and Governmental Debt Securities		0.0	[2]	0.0	[3]
Cash Equivalents		1.3	[2]	2.0	[3]
Total Fair Value of Assets		8.4		9.6
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)		8.4		9.6
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		11.7	[2]	10.9	[3]
Cash Equivalents		0.0	[2]	0.0	[3]
Total Fair Value of Assets		11.7		10.9
Liabilities [Abstract]
Deferred Compensation		16.0	[4]	16.1	[5]
U.S. Water Services Contingent Consideration		0.0	[4]	0.0	[5]
Total Fair Value of Liabilities		16.0		16.1
Total Net Fair Value of Assets (Liabilities)		(4.3)		(5.2)
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		25.0	[4]	36.6	[5]
Total Fair Value of Liabilities		25.0		36.6
Total Net Fair Value of Assets (Liabilities)		(25.0)		(36.6)
Activity in Level 3 [Roll Forward]
Beginning Balance		36.6		0.0
Ending Balance		25.0		36.6
Recurring Fair Value Measures [Member] | Level 3 [Member] | Recognition of U.S. Water Services Contingent Consideration [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3				(35.7)
Recurring Fair Value Measures [Member] | Level 3 [Member] | Accretion [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3	[6]	(2.8)		(2.4)
Recurring Fair Value Measures [Member] | Level 3 [Member] | Payments [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3		0.8		0.1
Recurring Fair Value Measures [Member] | Level 3 [Member] | Changes in Cash Flow Projections [Member]
Activity in Level 3 [Roll Forward]
Activity in Level 3		$ 13.6		$ 1.4

[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.