Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² land_parcel property renewal_option Lease	Dec. 31, 2016 USD ($) land_parcel	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Approximate square feet | ft²	14,600,000
Number of land parcels held for development | land_parcel	6	6
Investment in land held for development	$ 20,100	$ 20,100
Number of ground leases | property	109
Square feet subject to ground leases | ft²	8,900,000
Ground leases, weighted average remaining term	68 years 8 months 12 days
Approximately corporate office lease covers of rented space | ft²	36,653
Rental expense relating to the operating leases	$ 6,300	5,700	$ 5,100
Number of ground leases prepaid | Lease	48
Prepaid ground leases which represented rental expense	$ 500	$ 500	$ 500
Number of ground leases, excluding prepaid leases | Lease	61
First and Second Generation Leases [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Anticipated amount of investment	$ 27,800
Tennessee [Member]
Property, Plant and Equipment [Line Items]
Number of redevelopment properties under expansion | property	1
Total amount funded	$ 12,600
Tennessee [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	$ 3,200
Medical Office Building Expansion [Member] | North Carolina [Member]
Property, Plant and Equipment [Line Items]
Approximate square feet | ft²	38,000
Medical Office Building [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	$ 5,759
Estimated total investment	76,120
Estimated remaining fundings (unaudited)	$ 70,361
Approximate square feet | ft²	355,000
Total Funded During the Year	$ 5,073
Medical Office Building [Member] | North Carolina [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	3,487
Estimated total investment	12,000
Estimated remaining fundings (unaudited)	$ 8,513
Approximate square feet | ft²	204,000
Total Funded During the Year	$ 3,264
Medical Office Building [Member] | Washington [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	2,272
Estimated total investment	64,120
Estimated remaining fundings (unaudited)	$ 61,848
Approximate square feet | ft²	151,000
Total Funded During the Year	$ 1,809
Medical Office Building [Member] | Colorado [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	$ 14,600
Approximate square feet | ft²	99,957
Medical Office Building [Member] | Colorado [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	$ 2,800
Minimum [Member]
Property, Plant and Equipment [Line Items]
Ground leases, initial term	50 years
Ground leases, number of renewal options | renewal_option	1
Ground leases, renewal term	75 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Ground leases, initial term	75 years
Ground leases, number of renewal options | renewal_option	2
Ground leases, renewal term	100 years