Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2016		Dec. 31, 2015
Investments [Abstract]
Cash Equivalents		$ 2.5		$ 2.0
Liabilities [Abstract]
U.S. Water Services Contingent Consideration	[1]	36.7		36.6
Recurring Fair Value Measurements [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		7.0	[2]	7.6	[3]
Available-for-sale – Corporate Debt Securities		10.4	[2]	10.9	[3]
Cash Equivalents		2.5	[2]	2.0	[3]
Total Fair Value of Assets		19.9		20.5
Liabilities [Abstract]
Deferred Compensation		14.9	[4]	16.1	[5]
U.S. Water Services Contingent Consideration		36.7	[4]	36.6	[5]
Total Fair Value of Liabilities		51.6		52.7
Total Net Fair Value of Assets (Liabilities)		(31.7)		(32.2)
Fair Value Hierarchy Transfers, All Levels		0.0		0.0
Recurring Fair Value Measurements [Member] | Level 1 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		7.0	[2]	7.6	[3]
Available-for-sale – Corporate Debt Securities		0.0	[2]	0.0	[3]
Cash Equivalents		2.5	[2]	2.0	[3]
Total Fair Value of Assets		9.5		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)		9.5		9.6
Recurring Fair Value Measurements [Member] | Level 2 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		0.0	[2]	0.0	[3]
Available-for-sale – Corporate Debt Securities		10.4	[2]	10.9	[3]
Cash Equivalents		0.0	[2]	0.0	[3]
Total Fair Value of Assets		10.4		10.9
Liabilities [Abstract]
Deferred Compensation		14.9	[4]	16.1	[5]
U.S. Water Services Contingent Consideration		0.0	[4]	0.0	[5]
Total Fair Value of Liabilities		14.9		16.1
Total Net Fair Value of Assets (Liabilities)		(4.5)		(5.2)
Recurring Fair Value Measurements [Member] | Level 3 [Member]
Investments [Abstract]
Available-for-sale – Equity Securities		0.0	[2]	0.0	[3]
Available-for-sale – Corporate 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		36.7	[4]	36.6	[5]
Total Fair Value of Liabilities		36.7		36.6
Total Net Fair Value of Assets (Liabilities)		$ (36.7)		$ (36.6)

[1] Contingent Consideration relates to the estimated fair value of the earnings-based payment resulting from the U.S. Water Services acquisition. (See Note 3. Acquisitions and Note 5. 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.