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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract] |
Schedule of changes to asset retirement obligations | The following table shows changes to our AROs during the years ended December 31:
(in millions) |  | 2017 |  | 2016 |  | 2015 |
Balance as of January 1 |  | $ | 557.7 |  |  | $ | 571.2 |  |  | $ | 43.6 |  |
Integrys subsidiaries |  | — |  |  | — |  |  | 491.0 |  |
Accretion |  | 27.5 |  |  | 28.3 |  |  | 14.5 |  |
Additions and revisions to estimated cash flows |  | 26.5 |  | (1) | — |  |  | 35.5 |  | (2)
Liabilities settled |  | (38.0 | ) |  | (41.8 | ) |  | (13.4 | ) |
Balance as of December 31 |  | $ | 573.7 |  |  | $ | 557.7 |  |  | $ | 571.2 |  |
(1) | AROs increased $20.5 million in 2017 due to revisions made to estimated cash flows primarily for changes in the weighted average cost to retire natural gas distribution pipe at PGL and NSG. In addition, an ARO of $5.5 million was recorded related to the removal and dismantlement of WE's Rothschild Biomass Plant.
(2) | During 2015, an ARO of $16.1 million was recorded for fly-ash landfills located at generation facilities owned by WE and WPS. An ARO of $9.0 million was also recorded during 2015 for the Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals from Electric Utilities rule passed by the EPA in April 2015. In addition, AROs increased $10.4 million in 2015 due to revisions made to estimated cash flows primarily for changes in the weighted average cost to retire natural gas distribution pipe at PGL and NSG.

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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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