Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 2,104	$ 2,123	$ 2,031
Cash Flows From Investing Activities:
Other	7	5	32
Net Cash Provided by (Used in) Investing Activities	(2,205)	(2,141)	(1,976)
Cash flows from financing activities:
Dividends on common stock	(431)	(416)	(402)
Short-term debt, net	(74)	257	(413)
Issuances of Long-term debt	1,345	389	1,197
Capital issuance costs	(11)	(9)	(12)
Share-based payments	(39)	(83)	(12)
Net cash provided by (used in) financing activities	102	(265)	232
Net change in cash and cash equivalents	1	(283)	287
Cash and cash equivalents at beginning of year	9	292	5
Cash and cash equivalents at end of year	10	9	292
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	454	483	551
Cash Flows From Investing Activities:
Money pool advances, net	14	(27)	55
Intercompany notes receivable, net	275	(60)	(96)
Investments in subsidiaries	(151)	(123)	(509)
Other	6	2	(12)
Net Cash Provided by (Used in) Investing Activities	144	(208)	(562)
Cash flows from financing activities:
Dividends on common stock	(431)	(416)	(402)
Short-term debt, net	(124)	206	(284)
Money pool borrowings, net	(5)	19	14
Issuances of Long-term debt	0	0	700
Capital issuance costs	0	0	(6)
Share-based payments	(39)	(83)	(12)
Net cash provided by (used in) financing activities	(599)	(274)	10
Net change in cash and cash equivalents	(1)	1	(1)
Cash and cash equivalents at beginning of year	1	0	1
Cash and cash equivalents at end of year	0	1	0
Cash dividends received from consolidated subsidiaries	362	465	575
Noncash Investing activity- investment in subsidiaries	$ 0	$ 0	$ (38)