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	$ 363
Other comprehensive income (loss)	(10)
Ending balance at Mar. 31, 2016						(643)
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	426				426
Other comprehensive income (loss)	4					4
Issuance of common stock pursuant to employee stock options	111			111
Issuance of common stock pursuant to employee stock options (in shares)		7
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	(29)			(29)
Restricted stock surrendered for employees' tax liability (in shares)		(1)
Share-based compensation expense related to employee stock options and restricted stock rights	31			31
Dividends ($0.30 per common share)	(226)				(226)
Ending balance at Mar. 31, 2017	$ 9,436	$ 0	$ (5,563)	$ 10,555	$ 5,069	$ (625)
Balance (in shares) at Mar. 31, 2017		1,182	(429)