Commitments and Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating ground leases [Abstract]
Rental property expense	$ 2,500	$ 2,900	$ 3,400
Operating ground leases, scheduled future minimum payments [Abstract]
Operating leases, future minimum payments, 2018	2,099
Operating leases, future minimum payments, 2019	2,136
Operating leases, future minimum payments, 2020	2,175
Operating leases, future minimum payments, 2021	2,215
Operating leases, future minimum payments, 2022	2,257
Operating leases, future minimum payments, thereafter	88,532
Operating leases, total future minimum payments due	99,414
Lease and Contractual Commitments [Abstract]
Lease and contractual commitments	343,800
Accrued amount of lease and contractual commitments	67,500
Contingent Consideration [Abstract]
Asset acquisition, contingent consideration liability	750	0	$ 900
Asset Acquisition Contingent Consideration [Member]
Contingent Consideration [Abstract]
Asset acquisition, contingent consideration liability	$ 5,000	$ 4,200