CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) $ in Millions	Total	Common Stock and Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2017	$ 206	$ 583	$ (377)	$ 0
Issuance of common stock from exercise of stock options		6
Issuance of common stock for employee stock purchase plan		3
Stock-based compensation expense		19
Net income	112		112
Repurchase of common stock			(30)
Awards effectively repurchased for required employee withholding taxes			(10)
Other comprehensive income	0			0
Balance at Jun. 30, 2018	309	611	(302)	0
Balance at Mar. 31, 2018	262	595	(332)	(1)
Issuance of common stock from exercise of stock options		3
Issuance of common stock for employee stock purchase plan		3
Stock-based compensation expense		10
Net income	58		58
Repurchase of common stock			(22)
Awards effectively repurchased for required employee withholding taxes			(6)
Other comprehensive income	1			1
Balance at Jun. 30, 2018	309	611	(302)	0
Balance at Dec. 31, 2018	375	641	(266)	0
Issuance of common stock from exercise of stock options		2
Issuance of common stock for employee stock purchase plan		4
Stock-based compensation expense		20
Net income	109		109
Repurchase of common stock	(62)		(62)
Awards effectively repurchased for required employee withholding taxes			(10)
Other comprehensive income				1
Balance at Jun. 30, 2019	439	667	(229)	1
Balance at Mar. 31, 2019	406	651	(245)	0
Issuance of common stock from exercise of stock options		1
Issuance of common stock for employee stock purchase plan		4
Stock-based compensation expense		11
Net income	46		46
Repurchase of common stock			(24)
Awards effectively repurchased for required employee withholding taxes			(6)
Other comprehensive income				1
Balance at Jun. 30, 2019	$ 439	$ 667	$ (229)	$ 1