Commitments and contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Schedule of future minimum lease obligations under non-cancelable ground and other operating leases

Our rental expense attributable to continuing operations for the years ended December 31, 2018, 2017, and 2016, was approximately $15.8 million, $14.0 million, and $14.3 million, respectively. These rental expense amounts include certain operating leases for our headquarters and field offices and ground leases for 29 of our properties. Ground leases generally require fixed annual rent payments and may also include escalation clauses and renewal options. Future minimum lease obligations under non-cancelable ground and other operating leases as of December 31, 2018, were as follows (in thousands):

Year	Office Leases		Ground Leases		Total
2019	$	1,777	$	12,804	$	14,581
2020	326		12,631		12,957
2021	269		12,198		12,467
2022	—		12,302		12,302
2023	—		12,421		12,421
Thereafter	—		525,544		525,544
Total	$	2,372	$	587,900	$	590,272