Summary of Significant Accounting Policies Property, Plant, and Equipment (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Annual utility composite depreciation rate (as a percent)	3.00%	3.01%	2.93%
Software | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Software | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years