CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common Stock and Additional Paid-In Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2016	$ 35	$ 535	$ (500)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from exercise of stock options		11
Issuance of common stock for employee stock purchase plan		5
Stock-based compensation expense		32
Net income	178		178
Repurchase of common stock			(44)
Awards effectively repurchased for required employee withholding taxes			(11)
Ending balance at Dec. 31, 2017	206	583	(377)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from exercise of stock options		7
Issuance of common stock for employee stock purchase plan		7
Stock-based compensation expense		44
Net income	192		192
Repurchase of common stock			(61)
Awards effectively repurchased for required employee withholding taxes			(22)
Ending balance at Dec. 31, 2018	375	641	(266)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock from exercise of stock options		2
Issuance of common stock for employee stock purchase plan		9
Stock-based compensation expense		42
Net income	212		212
Repurchase of common stock			(140)
Awards effectively repurchased for required employee withholding taxes			(25)
Ending balance at Dec. 31, 2019	$ 475	$ 694	$ (219)