CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)
Balance (in shares) at Dec. 31, 2019			1,197,000	429,000
Beginning balance at Dec. 31, 2019	$ 12,805	$ (3)	$ 0	$ (5,563)	$ 11,174	$ 7,813	$ (3)	$ (619)
Components of comprehensive income:
Net income	505					505
Other comprehensive income (loss)	(9)							(9)
Issuance of common stock pursuant to employee stock options (in shares)			1,000
Issuance of common stock pursuant to employee stock options	27				27
Issuance of common stock pursuant to restricted stock units (in shares)			1,000
Issuance of common stock pursuant to restricted stock units	0
Restricted stock surrendered for employees' tax liability (in shares)			0
Restricted stock surrendered for employees’ tax liability	(31)				(31)
Share-based compensation expense related to employee stock options and restricted stock units	43				43
Dividends	(316)					(316)
Balance (in shares) at Mar. 31, 2020			1,199,000	429,000
Ending balance at Mar. 31, 2020	13,021		$ 0	$ (5,563)	11,213	7,999		(628)
Balance (in shares) at Dec. 31, 2020			1,203,000	429,000
Beginning balance at Dec. 31, 2020	15,037		$ 0	$ (5,563)	11,531	9,691		(622)
Components of comprehensive income:
Net income	619					619
Other comprehensive income (loss)	22							22
Issuance of common stock pursuant to employee stock options (in shares)			1,000
Issuance of common stock pursuant to employee stock options	33				33
Issuance of common stock pursuant to restricted stock units (in shares)			4,000
Issuance of common stock pursuant to restricted stock units	0
Restricted stock surrendered for employees' tax liability (in shares)			(2,000)
Restricted stock surrendered for employees’ tax liability	(165)				(165)
Share-based compensation expense related to employee stock options and restricted stock units	150				150
Dividends	(365)					(365)
Balance (in shares) at Mar. 31, 2021			1,206,000	429,000
Ending balance at Mar. 31, 2021	$ 15,331		$ 0	$ (5,563)	$ 11,549	$ 9,945		$ (600)