Schedule I - Condensed Financial Information - Condensed Statement of Income and Comprehensive Income (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements,Captions [Line Items]
Interest expense	$ (729)	$ (696)	$ (653)
NET INCOME	1,525	1,245	1,193
Comprehensive Income	$ 1,526	$ 1,252	$ 1,204
Net Income Per Share – Basic (dollars per share)	$ 4.97	$ 4.15	$ 4.07
Net Income Per Share – Diluted (dollars per share)	4.94	4.12	4.05
Dividends Declared Per Share (dollars per share)	$ 2.76	$ 2.68	$ 2.60
Average Number Of Shares Outstanding—Basic (In Millions)	307.1	300.4	293.0
Average Number Of Shares Outstanding—Diluted (In Millions)	308.8	301.9	294.4
Parent Company
Condensed Income Statements,Captions [Line Items]
Equity in earnings of subsidiaries	$ 1,544	$ 1,254	$ 1,195
Other income (deductions), net of taxes	31	32	27
Interest expense	(50)	(41)	(29)
NET INCOME	1,525	1,245	1,193
Comprehensive Income	$ 1,526	$ 1,252	$ 1,204
Average Number Of Shares Outstanding—Basic (In Millions)	307.1	300.4	293.0
Average Number Of Shares Outstanding—Diluted (In Millions)	308.8	301.9	294.4