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Summary of Significant Accounting Policies Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
PWGS | Minimum
Property, plant, and equipment
Estimated useful life	10 years
PWGS | Maximum
Property, plant, and equipment
Estimated useful life	45 years
ERGS | Minimum
Property, plant, and equipment
Estimated useful life	10 years
ERGS | Maximum
Property, plant, and equipment
Estimated useful life	55 years
Software | Minimum
Property, plant, and equipment
Estimated useful life	3 years
Software | Maximum
Property, plant, and equipment
Estimated useful life	15 years
WE
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.95%	3.00%	3.01%
WPS
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.55%	2.58%	1.30%
WG
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.30%	2.34%	2.36%
UMERC
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.46%
PGL
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	3.29%	3.31%	1.67%
NSG
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.43%	2.44%	1.22%
MERC
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.51%	2.53%	1.26%
MGU
Property, plant, and equipment
Annual utility composite depreciation rate (as a percent)	2.61%	2.63%	1.32%

X

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

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