Restructuring and other related costs - Restructuring expenses (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2020	Dec. 31, 2018	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Acceleration of rent expense	$ 16,106		$ 0
Acceleration of depreciation expense	5,377		0
Gain from extinguishment of lease liabilities	(1,866)		(7,733)
Employee severance and related expenses	600		0
Other costs, including other asset write-offs	1,959		1,751
Total	$ 22,176	$ 0	$ (5,982)	$ 0	$ 0