Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Schedule of inventory

Our inventory as of December 31 consisted of:

(in millions)	2016		2015
Materials and supplies	$	148.1	$	151.1
Fossil fuel	91.1		110.5
Natural gas in storage	31.8		40.0
Total	$	271.0	$	301.6

Allowance for funds used during construction

We recorded the following AFUDC for the years ended December 31:

(in millions)	2016		2015		2014
AFUDC – Debt	$	1.7	$	2.2	$	1.8
AFUDC – Equity	$	4.2	$	5.7	$	4.4

Schedule of assumptions used to estimate the fair value of stock options granted

The following table shows the estimated fair value per stock option granted to our employees along with the weighted-average assumptions used in the valuation models:

	2016			2015			2014
Non-qualified stock options granted *	92,880			495,550			864,860
Estimated fair value per non-qualified stock option	$	4.92		$	5.29		$	4.18
Risk-free interest rate	0.5% – 2.2%			0.1% – 2.1%			0.1% – 3.0%
Dividend yield	4.0		%	3.7		%	3.8		%
Expected volatility	18.0		%	18.0		%	18.0		%
Expected life (years)	5.8			5.8			5.8

* Effective January 1, 2016, certain of our employees were transferred into WBS.See Note 4, Related Parties, for more information.