Real Estate Assets and Construction in Progress (Summary of Properties Owned or Leased by the Company) (Details) $ in Thousands	Mar. 31, 2018 USD ($) property	Dec. 31, 2017 USD ($) property
Real Estate Properties [Line Items]
Land	$ 88,215	$ 88,216
Buildings and improvements	1,778,322	1,701,287
Construction in progress	636,054	567,819
Total cost	$ 2,502,591	$ 2,357,322
Number of facilities leased | property	11	11
Owned Properties | Atlanta, Georgia (Atlanta-Metro)
Real Estate Properties [Line Items]
Land	$ 20,416	$ 20,416
Buildings and improvements	461,668	452,836
Construction in progress	24,705	28,614
Total cost	506,789	501,866
Owned Properties | Irving Texas
Real Estate Properties [Line Items]
Land	8,606	8,606
Buildings and improvements	326,024	276,894
Construction in progress	69,317	86,320
Total cost	403,947	371,820
Owned Properties | Richmond, Virginia
Real Estate Properties [Line Items]
Land	2,180	2,180
Buildings and improvements	254,868	254,603
Construction in progress	63,007	61,888
Total cost	320,055	318,671
Owned Properties | Chicago, Illinois
Real Estate Properties [Line Items]
Land	9,400	9,400
Buildings and improvements	95,691	81,463
Construction in progress	128,587	135,479
Total cost	233,678	226,342
Owned Properties | Suwanee, Georgia (Atlanta-Suwanee)
Real Estate Properties [Line Items]
Land	3,521	3,521
Buildings and improvements	168,759	165,915
Construction in progress	3,266	3,620
Total cost	175,546	173,056
Owned Properties | Ashburn, Virginia
Real Estate Properties [Line Items]
Buildings and improvements	158
Construction in progress	170,741	106,952
Total cost	170,899	106,952
Owned Properties | Piscataway New Jersey
Real Estate Properties [Line Items]
Land	7,466	7,466
Buildings and improvements	84,122	83,251
Construction in progress	38,898	37,807
Total cost	130,486	128,524
Owned Properties | Santa Clara, California
Real Estate Properties [Line Items]
Buildings and improvements	100,450	100,028
Construction in progress	7,052	6,989
Total cost	107,502	107,017
Owned Properties | Dulles, Virginia
Real Estate Properties [Line Items]
Land	3,154	3,154
Buildings and improvements	77,569	76,239
Construction in progress	3,952	3,565
Total cost	84,675	82,958
Owned Properties | Sacramento, California
Real Estate Properties [Line Items]
Land	1,481	1,481
Buildings and improvements	64,438	64,251
Construction in progress	16	58
Total cost	65,935	65,790
Owned Properties | Fort Worth, Texas
Real Estate Properties [Line Items]
Land	9,079	9,079
Buildings and improvements	18,423	17,894
Construction in progress	34,887	33,774
Total cost	62,389	60,747
Owned Properties | Princeton, New Jersey
Real Estate Properties [Line Items]
Land	20,700	20,700
Buildings and improvements	32,987	32,948
Construction in progress	595	451
Total cost	54,282	54,099
Owned Properties | Phoenix, Arizona
Real Estate Properties [Line Items]
Construction in progress	28,269	27,402
Total cost	28,269	27,402
Owned Properties | Hillsboro, Oregon
Real Estate Properties [Line Items]
Construction in progress	31,593	29,278
Total cost	31,593	29,278
Owned Properties | Manassas, Virginia
Real Estate Properties [Line Items]
Construction in progress	24,717
Total cost	24,717
Owned Properties | Other
Real Estate Properties [Line Items]
Land	2,212	2,213
Buildings and improvements	35,666	35,505
Construction in progress	35	88
Total cost	37,913	37,806
Leased Properties | Owned Properties
Real Estate Properties [Line Items]
Buildings and improvements	57,499	59,460
Construction in progress	6,417	5,534
Total cost	$ 63,916	$ 64,994