Real Estate Assets and Construction in Progress (Summary of Properties Owned or Leased by the Company) (Details) $ in Thousands	Jun. 30, 2017 USD ($) property	Dec. 31, 2016 USD ($) property
Real Estate Properties [Line Items]
Land	$ 86,192	$ 74,130
Buildings and improvements	1,625,254	1,524,767
Construction in progress	363,449	365,960
Total cost	$ 2,074,895	$ 1,964,857
Number of facilities leased | property	13	13
Duluth, Georgia [Member] | Other Assets.
Real Estate Properties [Line Items]
Land	$ 1,900
Buildings and improvements	8,700
Construction in progress	100
Owned Properties [Member] | Atlanta, Georgia (Atlanta-Metro) [Member]
Real Estate Properties [Line Items]
Land	20,416	$ 15,397
Buildings and improvements	441,876	434,965
Construction in progress	29,982	32,422
Total cost	492,274	482,784
Owned Properties [Member] | Richmond, Virginia [Member]
Real Estate Properties [Line Items]
Land	2,180	2,180
Buildings and improvements	238,180	237,347
Construction in progress	74,191	70,580
Total cost	314,551	310,107
Owned Properties [Member] | Irving Texas [Member]
Real Estate Properties [Line Items]
Land	8,606	8,606
Buildings and improvements	245,887	204,713
Construction in progress	65,181	69,653
Total cost	319,674	282,972
Owned Properties [Member] | Suwanee, Georgia (Atlanta-Suwanee) [Member]
Real Estate Properties [Line Items]
Land	3,521	3,521
Buildings and improvements	168,544	171,376
Construction in progress	2,263	2,013
Total cost	174,328	176,910
Owned Properties [Member] | Chicago, Illinois [Member]
Real Estate Properties [Line Items]
Land	9,400	9,400
Buildings and improvements	80,426	45,848
Construction in progress	107,764	100,623
Total cost	197,590	155,871
Owned Properties [Member] | Piscataway New Jersey [Member]
Real Estate Properties [Line Items]
Land	7,466	7,466
Buildings and improvements	82,545	82,210
Construction in progress	33,353	17,261
Total cost	123,364	106,937
Owned Properties [Member] | Santa Clara, California [Member]
Real Estate Properties [Line Items]
Buildings and improvements	97,959	98,708
Construction in progress	8,709	7,078
Total cost	106,668	105,786
Owned Properties [Member] | Sacramento, California [Member]
Real Estate Properties [Line Items]
Land	1,481	1,481
Buildings and improvements	62,247	62,102
Construction in progress	1,792	390
Total cost	65,520	63,973
Owned Properties [Member] | Princeton, New Jersey [Member]
Real Estate Properties [Line Items]
Land	20,700	20,700
Buildings and improvements	32,782	32,788
Construction in progress	537	538
Total cost	54,019	54,026
Owned Properties [Member] | Fort Worth, Texas [Member]
Real Estate Properties [Line Items]
Land	9,079	136
Buildings and improvements	17,440	610
Construction in progress	29,147	49,116
Total cost	55,666	49,862
Owned Properties [Member] | Other [Member]
Real Estate Properties [Line Items]
Land	2,213	4,113
Buildings and improvements	35,379	37,810
Construction in progress	170	6,283
Total cost	37,762	48,206
Leased Properties [Member] | Owned Properties [Member]
Real Estate Properties [Line Items]
Land	1,130
Buildings and improvements	121,989
Construction in progress	10,360
Total cost	$ 133,479
Leased Properties [Member] | Owned Properties [Member] | Princeton, New Jersey [Member]
Real Estate Properties [Line Items]
Land		1,130
Buildings and improvements		116,290
Construction in progress		10,003
Total cost		$ 127,423