Consolidated Statements Of Stockholders' Equity - USD ($) $ in Millions	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Balance as of beginning-of-year at Dec. 31, 2016	$ 5,869	$ 7,043	$ 1,566
Stock compensation/issued for benefit plans	44
Net income (loss)		435		$ 435
Retirement of common stock/cancellation of shares	(74)	(126)
Common stock dividends declared		(65)
Other comprehensive income (loss), net of tax			281	281
Balance as of end-of-period at Mar. 31, 2017	5,839	7,287	1,847	14,973
Balance as of beginning-of-year at Dec. 31, 2017	5,693	8,399	3,230	17,322
Cumulative effect from adoption of new accounting standards at Dec. 31, 2017		(642)	642
Stock compensation/issued for benefit plans	7
Net income (loss)		367		367
Common stock dividends declared		(72)
Other comprehensive income (loss), net of tax			(1,639)	(1,639)
Balance as of end-of-period at Mar. 31, 2018	$ 5,700	$ 8,052	$ 2,233	$ 15,985