Other balance sheet items - Schedule of Other Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Pension obligation	$ 5,524		$ 5,524		$ 6,764
Lease liabilities	5,850		5,850		6,208
Unfavorable ground lease liability	2,170		2,170		2,187
Key money	16,257		16,257		8,225
Other	895		895		923
Total other liabilities	30,696		30,696		24,307
Service cost component of net periodic pension cost	200	$ 200	400	$ 400
Non-service cost components of net periodic pension (benefit) cost	$ 700	$ (200)	1,300	(200)
Receipt of key money			$ 8,500	$ 2,500	$ 6,500