Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2016

Accounting Policies [Abstract]

Schedule of Property, Plant and Equipment

Property, plant and equipment consisted of the following assets:

(millions)	Estimated Useful Lives	December 31, 2016			December 31, 2015
Electric generation	15-56 years	$	4,101.8		$	4,046.5
Electric transmission	28-77 years	836.8			711.2
Electric distribution	14-56 years		2,331.4			2,221.3
Gas transmission and distribution	16-77 years		1,429.1			1,326.1
General plant and other	3-43 years		438.8			373.5
Total cost			9,137.9			8,678.6
Less accumulated depreciation			(2,826.1	)		(2,676.8	)
Construction work in progress			891.5			771.1
Total property, plant and equipment, net		$	7,203.3		$	6,772.9