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Summary of Significant Accounting Policies (Tables) | 12 Months Ended
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Dec. 31, 2017
Accounting Policies [Abstract] |
Schedule of inventory | Our inventory as of December 31 consisted of:
(in millions) |  | 2017 |  | 2016
Natural gas in storage |  | $ | 209.0 |  |  | $ | 223.1 |
Materials and supplies |  | 211.2 |  |  | 206.5 |
Fossil fuel |  | 118.8 |  |  | 158.0 |
Total |  | $ | 539.0 |  |  | $ | 587.6 |
Schedule of annual utility composite depreciation rates | Annual utility composite depreciation rates are shown below:
Annual Utility Composite Depreciation Rates |  | 2017 |  | 2016 |  | 2015
WE |  | 2.95% |  | 3.00% |  | 3.01%
WPS (1) |  | 2.55% |  | 2.58% |  | 1.30%
WG |  | 2.30% |  | 2.34% |  | 2.36%
UMERC (2) |  | 2.46% |  | N/A |  | N/A
PGL (1) |  | 3.29% |  | 3.31% |  | 1.67%
NSG (1) |  | 2.43% |  | 2.44% |  | 1.22%
MERC (1) |  | 2.51% |  | 2.53% |  | 1.26%
MGU (1) |  | 2.61% |  | 2.63% |  | 1.32%
(1) | The rates shown for 2015 are for a partial year as a result of the acquisition of Integrys. The full year rate would be approximately double the rate shown.
(2) | UMERC became operational effective January 1, 2017. See Note 1(a), Nature of Operations, for more information.
Schedule of public utilities allowance for funds used during construction | Average AFUDC rates are shown below:
|  | 2017
|  | Average AFUDC Retail Rate |  | Average AFUDC Wholesale Rate
WE |  | 8.45% |  | 5.94%
WPS |  | 7.72% |  | 1.01%
WBS |  | 7.72% |  | N/A
WG |  | 8.33% |  | N/A
Allowance for funds used during construction | Our regulated utilities and WBS recorded the following AFUDC for the years ended December 31:
(in millions) |  | 2017 |  | 2016 |  | 2015
AFUDC – Debt |  | $ | 4.9 |  |  | $ | 10.9 |  |  | $ | 8.6 |
AFUDC – Equity |  | $ | 11.4 |  |  | $ | 25.1 |  |  | $ | 20.1 |
Schedule of assumptions used to estimate the fair value of stock options granted | The following table shows the estimated weighted-average fair value per stock option granted along with the weighted-average assumptions used in the valuation models:
|  | 2017 |  | 2016 |  | 2015
Stock options granted |  | 552,215 |  |  | 794,764 |  |  | 516,475 |
Estimated weighted-average fair value per stock option |  | $ | 7.45 |  |  | $ | 5.14 |  |  | $ | 5.29 |
Assumptions used to value the options: |  |  |  |  |  |
Risk-free interest rate |  | 0.7% – 2.5% |  |  | 0.4% – 2.2% |  |  | 0.1% – 2.1% |
Dividend yield |  | 3.5 | % |  | 4.0 | % |  | 3.7 | %
Expected volatility |  | 19.0 | % |  | 18.1 | % |  | 18.0 | %
Expected life (years) |  | 6.8 |  |  | 6.1 |  |  | 5.8 |

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