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Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2019
Accounting Policies [Abstract]  
Schedule of inventory Our inventory as of December 31 consisted of:
(in millions)
 
2019
 
2018
Materials and supplies
 
$
234.2

 
$
226.6

Natural gas in storage
 
227.7

 
232.9

Fossil fuel
 
87.9

 
88.7

Total
 
$
549.8

 
$
548.2
Schedule of annual utility composite depreciation rates Annual utility composite depreciation rates are shown below:
Annual Utility Composite Depreciation Rates
 
2019
 
2018
 
2017
WE
 
3.11%
 
3.18%
 
2.95%
WPS
 
2.44%
 
2.50%
 
2.55%
WG
 
2.29%
 
2.30%
 
2.30%
PGL
 
3.20%
 
3.25%
 
3.29%
NSG
 
2.48%
 
2.45%
 
2.43%
MERC *
 
2.33%
 
1.95%
 
2.51%
MGU
 
2.54%
 
2.61%
 
2.61%
UMERC
 
2.87%
 
2.50%
 
2.46%


*
The 2018 rate reflects the impact of a new depreciation study approved by the MPUC in May 2018. The rates approved were effective retroactive to January 2017. An approximate $1.4 million reduction in depreciation expense was recorded in 2018 related to this depreciation study.
Schedule of public utilities allowance for funds used during construction Average AFUDC rates are shown below:
 
 
2019
 
 
Average AFUDC Retail Rate
 
Average AFUDC Wholesale Rate
WE
 
8.45%
 
5.11%
WPS
 
7.72%
 
2.58%
WG
 
8.33%
 
N/A
UMERC
 
6.28%
 
N/A
WBS
 
7.72%
 
N/A


Our regulated utilities and WBS recorded the following AFUDC for the years ended December 31:
(in millions)
 
2019
 
2018
 
2017
AFUDC – Debt
 


 


 


   WE
 
$
1.5

 
$
1.5

 
$
1.2

   WPS
 
2.4

 
1.9

 
1.6

   WG
 
0.5

 
0.2

 
0.3

   UMERC
 
1.3

 
2.4

 
0.1

   WBS
 
0.1

 
0.2

 
1.1

Other
 
0.1

 
0.7

 
0.6

Total AFUDC – Debt
 
$
5.9

 
$
6.9

 
$
4.9

 
 
 
 
 
 
 
AFUDC – Equity
 


 


 


   WE
 
$
3.7

 
$
3.9

 
$
3.1

   WPS
 
5.7

 
4.6

 
4.1

   WG
 
1.3

 
0.6

 
0.9

   UMERC
 
3.3

 
5.4

 
0.2

   WBS
 
0.2

 
0.6

 
3.0

Other
 
0.2

 
0.1

 
0.1

Total AFUDC – Equity
 
$
14.4

 
$
15.2

 
$
11.4
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:
 
 
2019
 
2018
 
2017
Stock options granted
 
476,418

 
710,710

 
552,215

 
 
 
 
 
 
 
Estimated weighted-average fair value per stock option
 
$
8.60

 
$
7.71

 
$
7.45

 
 
 
 
 
 
 
Assumptions used to value the options:
 
 
 
 
 
 
Risk-free interest rate
 
2.5% – 2.7%

 
1.6% – 2.8%

 
0.7% – 2.5%

Dividend yield
 
3.6
%
 
3.5
%
 
3.5
%
Expected volatility
 
17.0
%
 
18.0
%
 
19.0
%
Expected life (years)
 
8.5

 
5.9

 
6.8