Stock-based compensation - Summary of Additional Information Relating to Performance-based and Market-based Stock Options (Details) - Performance-based and Market-based Stock Options - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0	$ 0	$ 1,168	$ 3,489
Intrinsic value of options exercised	$ 419	$ 609	$ 8,669	$ 42,874
Weighted-average grant date fair value of options granted (in USD per share)	$ 0.00	$ 0.00	$ 0.00	$ 10.65