Condensed Consolidating Financial Information (Condensed Consolidating Statements of Cash Flows) (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities	$ 664	$ 578
Capital investments	(684)	(619)
Other	9	13
Net cash used in investing activities	(675)	(606)
Payments on long-term debt	(1,191)	(287)
Payments on revolving credit facility	(645)
Borrowings under revolving credit facility	1,005
Preferred stock dividend	(27)
Other	(10)	3
Net cash used in financing activities	(868)	(284)
Increase (decrease) in cash and cash equivalents	(879)	(312)
Cash and cash equivalents at beginning of year	916	1,423
Cash and cash equivalents at end of period	37	1,111
Parent [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities	276	613
Capital investments	(6)	(7)
Other		1
Net cash used in investing activities	(6)	(6)
Intercompany activities	(287)	(634)
Payments on long-term debt	(1,191)	(287)
Payments on revolving credit facility	(645)
Borrowings under revolving credit facility	1,005
Preferred stock dividend	(27)
Other	(10)	3
Net cash used in financing activities	(1,155)	(918)
Increase (decrease) in cash and cash equivalents	(885)	(311)
Cash and cash equivalents at beginning of year	914	1,416
Cash and cash equivalents at end of period	29	1,105
Guarantors [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities	725	677
Capital investments	(678)	(609)
Other	9	12
Net cash used in investing activities	(669)	(597)
Intercompany activities	(50)	(81)
Net cash used in financing activities	(50)	(81)
Increase (decrease) in cash and cash equivalents	6	(1)
Cash and cash equivalents at beginning of year	2	7
Cash and cash equivalents at end of period	8	6
Non-Guarantors [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Capital investments		(3)
Net cash used in investing activities		(3)
Intercompany activities		3
Net cash used in financing activities		3
Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities	(337)	(712)
Intercompany activities	337	712
Net cash used in financing activities	$ 337	$ 712