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Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Investments [Abstract] |  |  |  |
Cash Equivalents | $ 2.0 |  | $ 3.8 |
Recurring Fair Value Measures [Member] |  |  |  |
Investments [Abstract] |  |  |  |
Available-for-sale – Equity Securities | 12.4 | [1] | 10.2 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 8.7 | [1] | 8.9 | [2]
Cash Equivalents | 2.0 | [1] | 3.8 | [2]
Total Fair Value of Assets | 23.1 |  | 22.9 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 20.2 | [3] | 18.2 | [4]
U.S. Water Services Contingent Consideration | 5.5 | [5] | 5.4 | [4]
Total Fair Value of Liabilities | 25.7 |  | 23.6 |
Total Net Fair Value of Assets (Liabilities) | (2.6) |  | (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 | 12.4 | [1] | 10.2 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 0.0 | [1] | 0.0 | [2]
Cash Equivalents | 2.0 | [1] | 3.8 | [2]
Total Fair Value of Assets | 14.4 |  | 14.0 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 0.0 | [3] | 0.0 | [4]
U.S. Water Services Contingent Consideration | 0.0 | [5] | 0.0 | [4]
Total Fair Value of Liabilities | 0.0 |  | 0.0 |
Total Net Fair Value of Assets (Liabilities) | 14.4 |  | 14.0 |
Recurring Fair Value Measures [Member] | Level 2 [Member] |  |  |  |
Investments [Abstract] |  |  |  |
Available-for-sale – Equity Securities | 0.0 | [1] | 0.0 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 8.7 | [1] | 8.9 | [2]
Cash Equivalents | 0.0 | [1] | 0.0 | [2]
Total Fair Value of Assets | 8.7 |  | 8.9 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 20.2 | [3] | 18.2 | [4]
U.S. Water Services Contingent Consideration | 0.0 | [5] | 0.0 | [4]
Total Fair Value of Liabilities | 20.2 |  | 18.2 |
Total Net Fair Value of Assets (Liabilities) | (11.5) |  | (9.3) |
Recurring Fair Value Measures [Member] | Level 3 [Member] |  |  |  |
Investments [Abstract] |  |  |  |
Available-for-sale – Equity Securities | 0.0 | [1] | 0.0 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 0.0 | [1] | 0.0 | [2]
Cash Equivalents | 0.0 | [1] | 0.0 | [2]
Total Fair Value of Assets | 0.0 |  | 0.0 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 0.0 | [3] | 0.0 | [4]
U.S. Water Services Contingent Consideration | 5.5 | [5] | 5.4 | [4]
Total Fair Value of Liabilities | 5.5 |  | 5.4 |
Total Net Fair Value of Assets (Liabilities) | (5.5) |  | (5.4) |
Activity in Level 3 [Roll Forward] |  |  |  |
Beginning Balance | 5.4 |  |  |
Ending Balance | 5.5 |  | $ 5.4 |
Recurring Fair Value Measures [Member] | Level 3 [Member] | Accretion [Member] |  |  |  |
Activity in Level 3 [Roll Forward] |  |  |  |
Activity in Level 3 | $ 0.1 |  |  |
[1] | Included in Other Investments on the Consolidated Balance Sheet.
[2] | Included in Other Investments on the Consolidated Balance Sheet.
[3] | Included in Other Non-Current Liabilities on the Consolidated Balance Sheet.
[4] | Included in Other Non-Current Liabilities on the Consolidated Balance Sheet.
[5] | Included in Other Current Liabilities on the Consolidated Balance Sheet.

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