Commitments and Contingencies - Construction Activity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) ft² property
Other Commitments [Line Items]
Number of Properties | property	201
Approximate Square Feet | ft²	14,600,000
Medical Office Building [Member]
Other Commitments [Line Items]
Construction in Progress Balance	$ 5,458
Total Funded During the Year	5,073
Total Amount Funded	5,759
Estimated Remaining Fundings (unaudited)	70,361
Estimated Total Investment (unaudited)	$ 76,120
Approximate Square Feet | ft²	355,000
North Carolina [Member] | Medical Office Building [Member]
Other Commitments [Line Items]
Number of Properties | property	1
Construction in Progress Balance	$ 3,487
Total Funded During the Year	3,264
Total Amount Funded	3,487
Estimated Remaining Fundings (unaudited)	8,513
Estimated Total Investment (unaudited)	$ 12,000
Approximate Square Feet | ft²	204,000
Washington [Member] | Medical Office Building [Member]
Other Commitments [Line Items]
Number of Properties | property	1
Construction in Progress Balance	$ 1,971
Total Funded During the Year	1,809
Total Amount Funded	2,272
Estimated Remaining Fundings (unaudited)	61,848
Estimated Total Investment (unaudited)	$ 64,120
Approximate Square Feet | ft²	151,000