Segment Information (Real Estate Assets by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Real estate assets	$ 5,705,931	$ 4,949,277
Carolinas/Richmond
Segment Reporting Information [Line Items]
Real estate assets	495,149	491,916
Chicago/Minneapolis
Segment Reporting Information [Line Items]
Real estate assets	545,919	557,782
Florida
Segment Reporting Information [Line Items]
Real estate assets	492,797	483,355
Houston
Segment Reporting Information [Line Items]
Real estate assets	509,100	456,675
Lehigh/Central PA
Segment Reporting Information [Line Items]
Real estate assets	1,151,148	1,121,115
Philadelphia
Segment Reporting Information [Line Items]
Real estate assets	286,568	279,338
Southeastern PA
Segment Reporting Information [Line Items]
Real estate assets	15,467	19,357
Southern California
Segment Reporting Information [Line Items]
Real estate assets	633,450	343,520
United Kingdom
Segment Reporting Information [Line Items]
Real estate assets	347,886	191,552
Other
Segment Reporting Information [Line Items]
Real estate assets	$ 1,228,447	$ 1,004,667