Consolidated statements of stockholders' equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Accumulated deficit
Beginning balance at Dec. 31, 2016	$ 140,862	$ 438	$ 700,871	$ (560,447)
Beginning balance (in shares) at Dec. 31, 2016		43,753,311
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	33,475			33,475
Stock-based compensation	13,474		13,474
Vesting of early exercised stock options	4,074	$ 15	4,059
Vesting of early exercised stock options (in shares)		1,522,826
Exercise of stock options, net	1,978	$ 10	1,968
Exercise of stock options (in shares)		1,039,493
Ending balance at Dec. 31, 2017	193,863	$ 463	720,372	(526,972)
Ending balance (in shares) at Dec. 31, 2017		46,315,630
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	15,525			15,525
Stock-based compensation	16,821		16,821
Exercise of stock options, net	3,176	$ 15	3,161
Exercise of stock options (in shares)		1,514,126
Ending balance at Dec. 31, 2018	229,331	$ 478	740,354	(511,501)
Ending balance (in shares) at Dec. 31, 2018		47,829,756
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(17,914)			(17,914)
Stock-based compensation	3,683		3,683
Exercise of stock options, net	$ 115	$ 5	110
Exercise of stock options (in shares)	62,450	458,964
Ending balance at Mar. 31, 2019	$ 215,215	$ 483	744,147	(529,415)
Ending balance (in shares) at Mar. 31, 2019		48,288,720
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	17,884			17,884
Stock-based compensation	15,488		15,488
Exercise of stock options, net	1,488	$ 12	1,476
Exercise of stock options (in shares)		1,150,490
Repurchase of common stock	$ (7,904)	$ (6)	(7,898)
Repurchase of common stock (in shares)	(564,468)	(564,468)
Ending balance at Mar. 31, 2020	$ 242,171	$ 489	$ 753,213	$ (511,531)
Ending balance (in shares) at Mar. 31, 2020		48,874,742