Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) ft² land_parcel renewal_option property Lease	Dec. 31, 2017 USD ($) ft² land_parcel	Dec. 31, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Approximate square feet | ft²	14,800,000
Number of land parcels held for development | land_parcel	7	6
Investment in land held for development	$ 24,600	$ 20,100
Number of ground leases | property	107
Square feet subject to ground leases | ft²	8,800,000
Ground leases, weighted average remaining term	53 years 10 months 24 days
Rental expense relating to the operating leases	$ 6,900	6,500	$ 5,800
Number of ground leases prepaid | Lease	47
Prepaid ground leases which represented rental expense	$ 500	$ 500	$ 500
Number of ground leases, excluding prepaid leases | Lease	60
First and Second Generation Leases [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Anticipated amount of investment	$ 29,200
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	33,407
Estimated total investment	76,120
Estimated remaining fundings (unaudited)	$ 42,713
Approximate square feet | ft²	189,000
Total Funded During the Year	$ 27,648
Medical Office Building [Member] | Nashville, Tennessee [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	6,100
Medical Office Building [Member] | Nashville, Tennessee [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	1,900
Medical Office Building [Member] | North Carolina [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	9,586
Estimated total investment	12,000
Estimated remaining fundings (unaudited)	$ 2,414
Approximate square feet | ft²	38,000
Total Funded During the Year	$ 6,099
Medical Office Building [Member] | Washington [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	23,821
Estimated total investment	64,120
Estimated remaining fundings (unaudited)	$ 40,299
Approximate square feet | ft²	151,000
Total Funded During the Year	$ 21,549
Medical Office Building [Member] | Colorado [Member]
Property, Plant and Equipment [Line Items]
Total amount funded	1,800
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	$ 100
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, renewal term	100 years