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EMPLOYEE BENEFITS-IMPACT OF ADOPTION OF ASU 2017-07 (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Other operation and maintenance | $ 511.9 | $ 504.5 | $ 2,056.1 | $ 2,171.3 | $ 1,710.7
Other income, net | 7.5 | 18.3 | 73.7 | 66.6 | 60.3
2017 Form 10-K Income Statement |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Other operation and maintenance |  |  | 2,047.0 | 2,185.5 | 1,709.3
Other income, net |  |  | 64.6 | 80.8 | 58.9
Accounting Standards Update 2017-07 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Non-service cost components of net benefit cost | $ (7.2) |  |  |  |
Accounting Standards Update 2017-07 | Impact of ASU 2017-07 |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |
Non-service cost components of net benefit cost |  | $ (2.6) |  |  |
Other operation and maintenance |  |  | 9.1 | (14.2) | 1.4
Other income, net |  |  | $ 9.1 | $ (14.2) | $ 1.4

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