Net income (loss) per share - Reconciliation of Numerator and Denominator in Basic and Diluted Net Income (Loss) Per Common Share Computations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income (loss)	$ (341)	$ 8,002	$ 6,517	$ 3,706	$ (17,914)	$ 9,672	$ 3,915	$ 1,248	$ 690	$ 17,884	$ 15,525	$ 33,475
Denominator:
Weighted average common shares outstanding - basic (in shares)					48,022,926				46,435,560	48,498,813	46,828,798	45,358,452
Dilutive common equivalent shares from equity awards (in shares)					0					2,318,330	2,439,818	4,016,306
Weighted average common shares outstanding - diluted (in shares)					48,022,926				49,302,771	50,817,143	49,268,616	49,374,758
Net income (loss) per share:
Basic (in USD per share)	$ (0.01)	$ 0.16	$ 0.13	$ 0.08	$ (0.37)	$ 0.20	$ 0.08	$ 0.03	$ 0.01	$ 0.37	$ 0.33	$ 0.74
Diluted (in USD per share)	$ (0.01)	$ 0.16	$ 0.13	$ 0.07	$ (0.37)	$ 0.20	$ 0.08	$ 0.03	$ 0.01	$ 0.35	$ 0.32	$ 0.68
Anti-dilutive securities excluded from diluted EPS:
Anti-dilutive securities excluded from diluted EPS, Total (in shares)					6,588,523					2,143,672	3,373,529	1,176,787