Earnings Per Share (EPS) And Dividends (Basic And Diluted Earnings Per Share Computation) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Diluted [Line Items]
Net Income									$ 1,574	$ 887	$ 1,679
Effect of Stock Based Compensation Awards, Basic									0	0	0
Total Shares, Basic	505,000	505,000	505,000	505,000	505,000	505,000	505,000	505,000	505,000	505,000	505,000
Effect of Stock Based Compensation Awards, Diluted									2,000	3,000	3,000
Total Shares, Diluted	508,000	507,000	507,000	508,000	508,000	508,000	508,000	508,000	507,000	508,000	508,000
Weighted Average Common Shares Outstanding Before Various Effects Basic									505,000	505,000	505,000
Weighted Average Common Shares Outstanding Before Various Effects Diluted									505,000	505,000	505,000
Earnings Per Share, Basic	$ 1.89	$ 0.78	$ 0.22	$ 0.23	$ (0.19)	$ 0.65	$ 0.37	$ 0.93	$ 3.12	$ 1.76	$ 3.32
Earnings Per Share, Diluted	$ 1.88	$ 0.78	$ 0.22	$ 0.22	$ (0.19)	$ 0.64	$ 0.37	$ 0.93	$ 3.10	$ 1.75	$ 3.30
Stock Options Excluded from Weighted Average Common Shares used for diluted EPS										400	500