Real Estate Assets and Construction in Progress (Summary of Properties Owned or Leased by the Company) (Details) $ in Thousands	Jun. 30, 2018 USD ($) property	Dec. 31, 2017 USD ($) property
Real Estate Properties [Line Items]
Land	$ 105,541	$ 88,216
Buildings, Improvements and Equipment	1,833,870	1,701,287
Construction in progress	649,766	567,819
Total cost	$ 2,589,177	$ 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, Improvements and Equipment	463,246	452,836
Construction in progress	30,336	28,614
Total cost	513,998	501,866
Owned Properties | Irving Texas
Real Estate Properties [Line Items]
Land	8,606	8,606
Buildings, Improvements and Equipment	325,474	276,894
Construction in progress	94,348	86,320
Total cost	428,428	371,820
Owned Properties | Richmond, Virginia
Real Estate Properties [Line Items]
Land	2,180	2,180
Buildings, Improvements and Equipment	253,122	254,603
Construction in progress	65,797	61,888
Total cost	321,099	318,671
Owned Properties | Chicago, Illinois
Real Estate Properties [Line Items]
Land	9,400	9,400
Buildings, Improvements and Equipment	98,366	81,463
Construction in progress	134,579	135,479
Total cost	242,345	226,342
Owned Properties | Suwanee, Georgia (Atlanta-Suwanee)
Real Estate Properties [Line Items]
Land	3,521	3,521
Buildings, Improvements and Equipment	168,813	165,915
Construction in progress	3,834	3,620
Total cost	176,168	173,056
Owned Properties | Ashburn, Virginia
Real Estate Properties [Line Items]
Land	17,326
Buildings, Improvements and Equipment	57,822
Construction in progress	122,651	106,952
Total cost	197,799	106,952
Owned Properties | Piscataway New Jersey
Real Estate Properties [Line Items]
Land	7,466	7,466
Buildings, Improvements and Equipment	84,745	83,251
Construction in progress	39,411	37,807
Total cost	131,622	128,524
Owned Properties | Santa Clara, California
Real Estate Properties [Line Items]
Buildings, Improvements and Equipment	97,610	100,028
Construction in progress	7,112	6,989
Total cost	104,722	107,017
Owned Properties | Dulles, Virginia
Real Estate Properties [Line Items]
Land	3,154	3,154
Buildings, Improvements and Equipment	76,521	76,239
Construction in progress	4,143	3,565
Total cost	83,818	82,958
Owned Properties | Sacramento, California
Real Estate Properties [Line Items]
Land	1,481	1,481
Buildings, Improvements and Equipment	64,476	64,251
Construction in progress	77	58
Total cost	66,034	65,790
Owned Properties | Fort Worth, Texas
Real Estate Properties [Line Items]
Land	9,078	9,079
Buildings, Improvements and Equipment	18,394	17,894
Construction in progress	36,094	33,774
Total cost	63,566	60,747
Owned Properties | Princeton, New Jersey
Real Estate Properties [Line Items]
Land	20,700	20,700
Buildings, Improvements and Equipment	33,367	32,948
Construction in progress	428	451
Total cost	54,495	54,099
Owned Properties | Phoenix, Arizona
Real Estate Properties [Line Items]
Construction in progress	28,691	27,402
Total cost	28,691	27,402
Owned Properties | Hillsboro, Oregon
Real Estate Properties [Line Items]
Construction in progress	34,420	29,278
Total cost	34,420	29,278
Owned Properties | Manassas, Virginia
Real Estate Properties [Line Items]
Construction in progress	39,151
Total cost	39,151
Owned Properties | Other
Real Estate Properties [Line Items]
Land	2,213	2,213
Buildings, Improvements and Equipment	35,542	35,505
Construction in progress	216	88
Total cost	37,971	37,806
Leased Properties | Owned Properties
Real Estate Properties [Line Items]
Buildings, Improvements and Equipment	56,372	59,460
Construction in progress	8,478	5,534
Total cost	$ 64,850	$ 64,994