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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%

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- 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. + References No definition available. + Details
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- Definition Composite depreciation rate for public utility plants in service. + References No definition available. + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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