Segment Information - Real Estate Assets by Segment (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Real estate assets	$ 5,865,562	$ 5,892,429
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Real estate assets	322,707	309,822
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Real estate assets	387,182	383,494
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Real estate assets	483,432	398,080
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,096,195	958,886
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Real estate assets	322,627	302,830
North America [Member] | Operating Segments [Member] | Orlando [Member]
Segment Reporting Information [Line Items]
Real estate assets	119,356	139,218
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Real estate assets	282,159	255,313
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Real estate assets	123,641	238,670
North America [Member] | Operating Segments [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Real estate assets	389,534	378,398
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	410,195	636,280
North America [Member] | Operating Segments [Member] | Tampa [Member]
Segment Reporting Information [Line Items]
Real estate assets	306,822	317,106
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,447,322	1,391,744
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Real estate assets	$ 174,390	$ 182,588