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Income Taxes - Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income taxes |  |  |  |
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent |  | 35.00% | 35.00% | 35.00%
Non-current |  |  |  |
Tax gross up - regulatory items |  | $ 585.8 | $ 0.0 |
Future tax benefits |  | 303.9 | 430.4 |
Employee benefits and compensation |  | 164.2 | 222.0 |
Deferred revenues |  | 128.8 | 207.2 |
Property-related |  | 24.4 | 54.5 |
Other |  | 185.0 | 230.6 |
Total deferred tax assets |  | 1,392.1 | 1,144.7 |
Valuation allowance |  | (15.7) | (15.0) |
Net deferred tax assets |  | 1,376.4 | 1,129.7 |
Non-current |  |  |  |
Property-related |  | 3,464.6 | 4,979.3 |
Investment in transmission affiliate |  | 321.2 | 476.9 |
Employee benefits and compensation |  | 285.8 | 401.6 |
Deferred transmission costs |  | 60.1 | 93.1 |
Other |  | 244.5 | 325.4 |
Total deferred tax liabilities |  | 4,376.2 | 6,276.3 |
Deferred tax liability, net |  | 2,999.8 | $ 5,146.6 |
Utility operations |  |  |  |
Income taxes |  |  |  |
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation |  | 2,657.0 |  |
Non-utility operations |  |  |  |
Income taxes |  |  |  |
Estimated tax benefit related to the remeasurement of deferred taxes from tax legislation |  | $ 206.7 |  |
Federal tax rate change due to the tax legislation |  |  |  |
Income taxes |  |  |  |
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent | 21.00% |  |  |

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