CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (Unaudited) - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2015						$ (633)
Components of comprehensive income:
Net income	$ 514
Other comprehensive loss	(16)
Ending balance at Jun. 30, 2016						(649)
Beginning balance at Dec. 31, 2016	9,119	$ 0	$ (5,563)	$ 10,442	$ 4,869	(629)
Balance (in shares) at Dec. 31, 2016		1,174	(429)
Components of comprehensive income:
Net income	669				669
Other comprehensive loss	(11)					(11)
Issuance of common stock pursuant to employee stock options	130			130
Issuance of common stock pursuant to employee stock options (in shares)		8
Issuance of common stock pursuant to restricted stock units	0			0
Issuance of common stock pursuant to restricted stock units (in shares)		2
Restricted stock surrendered for employees’ tax liability	(37)			(37)
Restricted stock surrendered for employees' tax liability (in shares)		(1)
Share-based compensation expense related to employee stock options and restricted stock rights	71			71
Dividends ($0.30 per common share)	(226)				(226)
Ending balance at Jun. 30, 2017	$ 9,715	$ 0	$ (5,563)	$ 10,606	$ 5,312	$ (640)
Balance (in shares) at Jun. 30, 2017		1,183	(429)