Other assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Acquired below-market ground leases	$ 12,684	$ 12,913
Acquired in-place leases	64,979	63,408
Deferred compensation plan assets	15,534	11,632
Deferred financing costs	10,525	14,239
Deposits	10,576	3,302
Furniture, fixtures, and equipment	11,070	12,839
Interest rate hedge assets	5,260	4,115
Net investment in direct financing leases	38,382	37,297
Notes receivable	614	694
Prepaid expenses	10,972	9,724
Property, plant, and equipment	32,073	19,891
Other Assets	15,784	11,143
Total	$ 228,453	$ 201,197