Income taxes - Schedule of Reconciliation of Federal Statutory Rate to Company's Effective Tax Rate (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory rate	21.00%	21.00%	21.00%	35.00%
Federal tax deferred rate change	0.000	0.000	0.000	(0.538)
State tax, net of federal benefit	1.20%	3.70%	2.60%	0.60%
State tax deferred rate change, net of federal benefit	0.00%	0.10%	0.90%	0.90%
Nondeductible business expenses	(0.10%)	0.80%	1.40%	0.00%
Provision-to-return adjustment	0.00%	0.00%	(3.90%)	0.00%
Uncertain tax positions	(0.10%)	(0.20%)	(1.30%)	(1.70%)
Stock based compensation	(6.10%)	(0.40%)	(8.60%)	(28.10%)
Others	(0.50%)	0.70%	1.40%	(1.90%)
Effective tax rate	15.40%	25.70%	13.50%	(49.00%)