Other balance sheet items - Schedule of Other Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Pension obligation	$ 5,563		$ 5,563		$ 6,764
Operating Lease, Liability, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilities		us-gaap:OtherLiabilities		us-gaap:OtherLiabilities
Lease liabilities	$ 4,937		$ 4,937		$ 6,208
Unfavorable ground lease liability	2,114		2,114		2,187
Key money	16,024		16,024		8,225
Other	883		883		923
Total other liabilities	29,521		29,521		24,307
Service cost component of net periodic pension cost	200	$ 200	600	$ 600
Non-service cost components of net periodic pension (benefit) cost	$ 300	$ (200)	1,600	(400)
Receipt of key money			$ 8,500	$ 2,500	$ 6,500