Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Schedule of Property, Plant and Equipment

Property, plant and equipment consisted of the following assets:

(millions)	Estimated Useful Lives	December 31, 2017			December 31, 2016
Electric generation	21-56 years	$	4,766		$	4,102
Electric transmission	28-77 years		859			837
Electric distribution	14-56 years		2,437			2,331
Gas transmission and distribution	15-77 years		1,534			1,429
General plant and other	8-43 years		579			438
Total cost			10,175			9,137
Less accumulated depreciation			(2,994	)		(2,826	)
Construction work in progress			263			892
Total property, plant and equipment, net		$	7,444		$	7,203