Real Estate Assets and Construction in Progress (Summary of Properties Owned or Leased by the Company) (Details) $ in Thousands	Dec. 31, 2017 USD ($) property	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) property
Real Estate Properties [Line Items]
Land	$ 88,216		$ 74,130
Buildings and improvements	1,701,287		1,524,767
Construction in progress	567,819		365,960
Total cost	$ 2,357,322		$ 1,964,857
Number of facilities leased | property	11		12
Duluth, Georgia | Other Assets.
Real Estate Properties [Line Items]
Land		$ 1,900
Buildings and improvements		8,700
Construction in progress		$ 100
Owned Properties | Atlanta, Georgia (Atlanta-Metro)
Real Estate Properties [Line Items]
Land	$ 20,416		$ 15,397
Buildings and improvements	452,836		434,965
Construction in progress	28,614		32,422
Total cost	501,866		482,784
Owned Properties | Irving Texas
Real Estate Properties [Line Items]
Land	8,606		8,606
Buildings and improvements	276,894		204,713
Construction in progress	86,320		69,653
Total cost	371,820		282,972
Owned Properties | Richmond, Virginia
Real Estate Properties [Line Items]
Land	2,180		2,180
Buildings and improvements	254,603		237,347
Construction in progress	61,888		70,580
Total cost	318,671		310,107
Owned Properties | Chicago, Illinois
Real Estate Properties [Line Items]
Land	9,400		9,400
Buildings and improvements	81,463		45,848
Construction in progress	135,479		100,623
Total cost	226,342		155,871
Owned Properties | Suwanee, Georgia (Atlanta-Suwanee)
Real Estate Properties [Line Items]
Land	3,521		3,521
Buildings and improvements	165,915		171,376
Construction in progress	3,620		2,013
Total cost	173,056		176,910
Owned Properties | Piscataway New Jersey
Real Estate Properties [Line Items]
Land	7,466		7,466
Buildings and improvements	83,251		82,210
Construction in progress	37,807		17,261
Total cost	128,524		106,937
Owned Properties | Santa Clara, California
Real Estate Properties [Line Items]
Buildings and improvements	100,028		98,708
Construction in progress	6,989		7,078
Total cost	107,017		105,786
Owned Properties | Sacramento, California
Real Estate Properties [Line Items]
Land	1,481		1,481
Buildings and improvements	64,251		62,102
Construction in progress	58		390
Total cost	65,790		63,973
Owned Properties | Fort Worth, Texas
Real Estate Properties [Line Items]
Land	9,079		136
Buildings and improvements	17,894		610
Construction in progress	33,774		49,116
Total cost	60,747		49,862
Owned Properties | Princeton, New Jersey
Real Estate Properties [Line Items]
Land	20,700		20,700
Buildings and improvements	32,948		32,788
Construction in progress	451		538
Total cost	54,099		54,026
Owned Properties | Dulles, Virginia
Real Estate Properties [Line Items]
Land	3,154
Buildings and improvements	76,239
Construction in progress	3,565
Total cost	82,958
Owned Properties | Ashburn, Virginia
Real Estate Properties [Line Items]
Construction in progress	106,952
Total cost	106,952
Owned Properties | Phoenix, Arizona
Real Estate Properties [Line Items]
Construction in progress	27,402
Total cost	27,402
Owned Properties | Hillsboro, Oregon
Real Estate Properties [Line Items]
Construction in progress	29,278
Total cost	29,278
Owned Properties | Other
Real Estate Properties [Line Items]
Land	2,213		4,113
Buildings and improvements	35,505		37,810
Construction in progress	88		6,283
Total cost	37,806		48,206
Leased Properties | Owned Properties
Real Estate Properties [Line Items]
Land			1,130
Buildings and improvements	59,460		116,290
Construction in progress	5,534		10,003
Total cost	$ 64,994		$ 127,423