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Fair Value - Recurring Fair Value Measures (Details) - USD ($) $ in Millions | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Investments [Abstract] |  |  |  |
Cash Equivalents | $ 1.5 |  | $ 3.8 |
Recurring Fair Value Measures [Member] |  |  |  |
Investments [Abstract] |  |  |  |
Available-for-sale – Equity Securities | 12.8 | [1] | 10.2 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 8.6 | [1] | 8.9 | [2]
Cash Equivalents | 1.5 | [1] | 3.8 | [2]
Total Fair Value of Assets | 22.9 |  | 22.9 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 20.5 | [3] | 18.2 | [4]
U.S. Water Services Contingent Consideration | 5.6 | [5] | 5.4 | [4]
Total Fair Value of Liabilities | 26.1 |  | 23.6 |
Total Net Fair Value of Assets (Liabilities) | (3.2) |  | (0.7) |
Activity in Level 3 [Roll Forward] |  |  |  |
Transfers In or Out, All Levels | 0.0 |  | 0.0 |
Recurring Fair Value Measures [Member] | Level 1 [Member] |  |  |  |
Investments [Abstract] |  |  |  |
Available-for-sale – Equity Securities | 12.8 | [1] | 10.2 | [2]
Available-for-sale – Corporate and Governmental Debt Securities | 0.0 | [1] | 0.0 | [2]
Cash Equivalents | 1.5 | [1] | 3.8 | [2]
Total Fair Value of Assets | 14.3 |  | 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) | 14.3 |  | 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 | 8.6 | [1] | 8.9 | [2]
Cash Equivalents | 0.0 | [1] | 0.0 | [2]
Total Fair Value of Assets | 8.6 |  | 8.9 |
Liabilities [Abstract] |  |  |  |
Deferred Compensation | 20.5 | [3] | 18.2 | [4]
U.S. Water Services Contingent Consideration | 0.0 | [5] | 0.0 | [4]
Total Fair Value of Liabilities | 20.5 |  | 18.2 |
Total Net Fair Value of Assets (Liabilities) | (11.9) |  | (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 | 5.6 | [5] | 5.4 | [4]
Total Fair Value of Liabilities | 5.6 |  | 5.4 |
Total Net Fair Value of Assets (Liabilities) | (5.6) |  | (5.4) |
Activity in Level 3 [Roll Forward] |  |  |  |
Beginning Balance | 5.4 |  |  |
Ending Balance | 5.6 |  | $ 5.4 |
Recurring Fair Value Measures [Member] | Level 3 [Member] | Accretion [Member] |  |  |  |
Activity in Level 3 [Roll Forward] |  |  |  |
Accretion | $ 0.2 |  |  |
[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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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (available-for-sale), classified as noncurrent. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562 + Details
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