Summary of Significant Accounting and Reporting Policies (Goodwill and Intangible Assets) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill and Intangible Assets [Line Items]
Goodwill	$ 779	$ 778
Other intangible assets not subject to amortization, primarily land easements	143	143
Other intangible assets:
Total	1,225	1,184
Accumulated amortization	(115)	(120)
Total other intangible assets subject to amortization - net	1,110	1,064
Intangible assets, amortization [Abstract]
Amortization expense	35	17	$ 15
Amortization Expense 2017	31
Amortization Expense 2018	46
Amortization Expense 2019	39
Amortization Expense 2020	26
Amortization Expense 2021	$ 19
Customer relationships associated with gas infrastructure
Other intangible assets:
Weighted average useful lives (years)	41 years
Total	$ 700	720
Purchased power agreements
Other intangible assets:
Weighted average useful lives (years)	22 years
Total	$ 444	328
Other, primarily transmission and development rights and customer lists
Other intangible assets:
Weighted average useful lives (years)	19 years
Total	$ 81	136
Gas Infrastructure, primarily Texas pipelines [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	641	635
Customer Supply [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	72	72
Generation Assets [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	38	43
Other Goodwill Assets [Member]
Goodwill and Intangible Assets [Line Items]
Goodwill	$ 28	$ 28