Summary of Significant Accounting and Reporting Policies (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill and Intangible Assets [Line Items]
Goodwill	$ 764	$ 779
Other intangible assets not subject to amortization, primarily land easements	138	143
Other intangible assets:
Total	1,300	1,225
Accumulated amortization	(151)	(115)
Total other intangible assets subject to amortization - net	1,149	1,110
Intangible assets, amortization [Abstract]
Amortization expense	35	35	$ 17
Amortization Expense 2017	36
Amortization Expense 2018	35
Amortization Expense 2019	35
Amortization Expense 2020	35
Amortization Expense 2021	$ 35
Customer relationships associated with gas infrastructure [Member]
Other intangible assets:
Weighted average useful lives (years)	41 years
Total	$ 700	700
Purchased Power Agreements [Member]
Other intangible assets:
Weighted average useful lives (years)	22 years
Total	$ 521	444
Other, primarily transmission and development rights and customer lists [Member]
Other intangible assets:
Weighted average useful lives (years)	23 years
Total	$ 79	81
Gas Infrastructure, primarily Texas pipelines [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	641	641
Customer Supply [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	72	72
Generation Assets [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	40	38
Other Goodwill Assets [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	$ 11	$ 28