CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - 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	$ 8,068	$ 0	$ (5,637)	$ 10,242	$ 4,096	$ (633)
Balance (in shares) at Dec. 31, 2015		1,163	429
Components of comprehensive income:
Net income	966				966
Other comprehensive income (loss)	4					4
Issuance of common stock pursuant to employee stock options	105			105
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)		7
Restricted stock surrendered for employees’ tax liability	(116)			(116)
Restricted stock surrendered for employees' tax liability (in shares)		(3)
Share-based compensation expense related to employee stock options and restricted stock units	135			135
Share-based compensation assumed in acquisition	76			76
Dividends ($0.34, $0.30, and $0.26 per common share at December 31, 2018, 2017, and 2016, respectively)	(193)				(193)
Indemnity on tax attributes assumed in connection with the Purchase Transaction	74		$ 74
Ending 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	273				273
Other comprehensive income (loss)	(9)					(9)
Issuance of common stock pursuant to employee stock options	178			178
Issuance of common stock pursuant to employee stock options (in shares)		11
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	(54)			(54)
Restricted stock surrendered for employees' tax liability (in shares)		(1)
Share-based compensation expense related to employee stock options and restricted stock units	181			181
Dividends ($0.34, $0.30, and $0.26 per common share at December 31, 2018, 2017, and 2016, respectively)	(226)				(226)
Ending balance at Dec. 31, 2017	9,462	$ 0	$ (5,563)	10,747	4,916	(638)
Balance (in shares) at Dec. 31, 2017		1,186	429
Components of comprehensive income:
Net income	1,813				1,813
Other comprehensive income (loss)	34					34
Issuance of common stock pursuant to employee stock options	98			98
Issuance of common stock pursuant to employee stock options (in shares)		5
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	(93)			(93)
Restricted stock surrendered for employees' tax liability (in shares)		(1)
Share-based compensation expense related to employee stock options and restricted stock units	211			211
Dividends ($0.34, $0.30, and $0.26 per common share at December 31, 2018, 2017, and 2016, respectively)	(259)				(259)
Ending balance at Dec. 31, 2018	$ 11,357	$ 0	$ (5,563)	$ 10,963	$ 6,558	$ (601)
Balance (in shares) at Dec. 31, 2018		1,192	429