Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2020

Accounting Policies [Abstract]

Schedule of inventory

Our inventory as of December 31 consisted of:

(in millions)	2020		2019
Natural gas in storage	$	224.9	$	227.7
Materials and supplies	218.1		234.2
Fossil fuel	85.6		87.9
Total	$	528.6	$	549.8

Schedule of annual utility composite depreciation rates

Annual utility composite depreciation rates are shown below:

Annual Utility Composite Depreciation Rates	2020	2019	2018
WE	3.19%	3.11%	3.18%
WPS	2.63%	2.44%	2.50%
WG	2.33%	2.29%	2.30%
PGL	3.16%	3.20%	3.25%
NSG	2.48%	2.48%	2.45%
MERC (1)	2.47%	2.33%	1.95%
MGU	2.67%	2.54%	2.61%
UMERC	2.97%	2.87%	2.50%

(1)    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:

	2020
	Average AFUDC Retail Rate	Average AFUDC Wholesale Rate
WE	8.68%	5.39%
WPS	7.55%	5.59%
WG	8.32%	N/A
UMERC	6.28%	N/A
WBS	7.55%	N/A

Our regulated utilities and WBS recorded the following AFUDC for the years ended December 31:

(in millions)	2020		2019		2018
AFUDC – Debt
WE	$	2.6	$	1.5	$	1.5
WPS	4.6		2.4		1.9
WG	0.6		0.5		0.2
UMERC	—		1.3		2.4
WBS	0.1		0.1		0.2
Other	0.1		0.1		0.7
Total AFUDC – Debt	$	8.0	$	5.9	$	6.9
AFUDC – Equity
WE	$	7.0	$	3.7	$	3.9
WPS	11.8		5.7		4.6
WG	1.6		1.3		0.6
UMERC	0.1		3.3		5.4
WBS	0.2		0.2		0.6
Other	0.2		0.2		0.1
Total AFUDC – Equity	$	20.9	$	14.4	$	15.2

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:

	2020			2019			2018
Stock options granted	554,594			476,418			710,710
Estimated weighted-average fair value per stock option	$	10.94		$	8.60		$	7.71
Assumptions used to value the options:
Risk-free interest rate	0.2% – 1.9%			2.5% – 2.7%			1.6% – 2.8%
Dividend yield	3.0		%	3.6		%	3.5		%
Expected volatility	16.3		%	17.0		%	18.0		%
Expected life (years)	8.6			8.5			5.9