Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) ft² property Lease renewal_option	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 USD ($)
Property, Plant and Equipment [Line Items]
Number of Properties | property	202
Land held for development	$ 20,123	$ 17,452
Approximate square feet | ft²	14,600,000
Investment in land held for development	$ 20,100	17,500
Number of ground leases | property	99
Square feet subject to ground leases | ft²	8,000,000
Ground leases, weighted average remaining term	69 years 4 months 24 days
Approximately corporate office lease covers of rented space | ft²	36,653
Percentage on increases of annual base rent on the corporate office lease	3.25%
Rental expense relating to the operating leases	$ 5,700	5,100	$ 4,900
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	52
First and Second Generation Leases [Member] | Tenant Improvement Allowances [Member]
Property, Plant and Equipment [Line Items]
Anticipated amount of investment	$ 19,100
Corporate Office Lease [Member]
Property, Plant and Equipment [Line Items]
Corporate lease expiration date	Oct. 31, 2020
Tennessee [Member]
Property, Plant and Equipment [Line Items]
Number of Properties | property		2
Number of redevelopment properties under expansion | property		1
Total amount funded	$ 50,600
Land held for development	$ 3,100
Medical Office Building [Member] | Tennessee [Member]
Property, Plant and Equipment [Line Items]
Number of Properties | property	2
Total amount funded	$ 50,612
Estimated total investment	56,620
Estimated remaining fundings (unaudited)	$ 6,008
Approximate square feet | ft²	294,000
Medical Office Building [Member] | Colorado [Member]
Property, Plant and Equipment [Line Items]
Number of Properties | property	1
Total amount funded	$ 11,655
Estimated total investment	26,500
Estimated remaining fundings (unaudited)	$ 14,845
Approximate square feet | ft²	98,000
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