Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Initial Cost
Land	$ 146,197
Building and Improvements	428,403
Costs Capitalized Subsequent to Acquisition	1,904,198
Gross Amount Carried As of Year End
Land	94,593
Building and Improvements	2,384,205
Total	2,478,798	$ 2,083,205	$ 1,821,217	$ 1,643,576
Accumulated Depreciation	720,498	$ 590,784	$ 473,141	$ 369,303
Aggregate cost of the total properties for federal income tax purposes	2,133,900
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	92,464
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	120,158
Total	125,312
Accumulated Depreciation	66,223
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	66,321
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	115,843
Total	121,336
Accumulated Depreciation	52,821
CH2 Chicago IL
Initial Cost
Land	4,383
Costs Capitalized Subsequent to Acquisition	95,591
Gross Amount Carried As of Year End
Building and Improvements	99,974
Total	99,974
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	5,311
Gross Amount Carried As of Year End
Building and Improvements	10,108
Total	10,108
Accumulated Depreciation	7,404
DC2 Washington DC
Initial Cost
Costs Capitalized Subsequent to Acquisition	22,560
Gross Amount Carried As of Year End
Building and Improvements	22,560
Total	22,560
Accumulated Depreciation	2,336
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	32,647
Gross Amount Carried As of Year End
Building and Improvements	32,684
Total	32,684
Accumulated Depreciation	9,238
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	2,432
Gross Amount Carried As of Year End
Building and Improvements	2,436
Total	2,436
Accumulated Depreciation	1,081
LA1 Los Angeles CA [Member]
Initial Cost
Building and Improvements	41,657
Costs Capitalized Subsequent to Acquisition	61,776
Gross Amount Carried As of Year End
Building and Improvements	103,433
Total	103,433
Accumulated Depreciation	57,848
LA2 Los Angeles CA [Member]
Initial Cost
Land	18,672
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	177,460
Gross Amount Carried As of Year End
Land	18,672
Building and Improvements	271,574
Total	290,246
Accumulated Depreciation	94,256
LA3 Los Angeles CA
Initial Cost
Land	9,795
Costs Capitalized Subsequent to Acquisition	48,986
Gross Amount Carried As of Year End
Building and Improvements	58,781
Total	58,781
LA4 Los Angeles CA
Initial Cost
Building and Improvements	2,533
Costs Capitalized Subsequent to Acquisition	382
Gross Amount Carried As of Year End
Building and Improvements	2,915
Total	2,915
Accumulated Depreciation	890
MI1 Miami FL [Member]
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	5,299
Gross Amount Carried As of Year End
Land	728
Building and Improvements	14,624
Total	15,352
Accumulated Depreciation	4,938
NY1 New York NY [Member]
Initial Cost
Costs Capitalized Subsequent to Acquisition	43,340
Gross Amount Carried As of Year End
Building and Improvements	43,340
Total	43,340
Accumulated Depreciation	29,633
NY2 Secaucus NJ [Member]
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	148,046
Gross Amount Carried As of Year End
Land	2,729
Building and Improvements	168,677
Total	171,406
Accumulated Depreciation	37,441
SV1 San Jose CA [Member]
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	44,091
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	135,810
Total	142,673
Accumulated Depreciation	49,727
SV2 Milpitas CA [Member]
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	28,342
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	33,388
Total	38,474
Accumulated Depreciation	23,578
SV3 Santa Clara CA [Member]
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	50,985
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	54,657
Total	58,629
Accumulated Depreciation	44,812
SV4 Santa Clara CA [Member]
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	99,272
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	103,119
Total	107,620
Accumulated Depreciation	53,801
SV5 Santa Clara CA [Member]
Initial Cost
Land	2,572
Costs Capitalized Subsequent to Acquisition	23,577
Gross Amount Carried As of Year End
Land	2,544
Building and Improvements	23,605
Total	26,149
Accumulated Depreciation	3,844
SV6 Santa Clara CA
Initial Cost
Land	4,741
Costs Capitalized Subsequent to Acquisition	36,758
Gross Amount Carried As of Year End
Land	4,825
Building and Improvements	36,674
Total	41,499
Accumulated Depreciation	3,377
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	246,559
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	246,757
Total	250,352
Accumulated Depreciation	44,557
SV8 Santa Clara CA
Initial Cost
Land	12,158
Costs Capitalized Subsequent to Acquisition	184,198
Gross Amount Carried As of Year End
Land	8,575
Building and Improvements	187,781
Total	196,356
Accumulated Depreciation	1,989
SV9 Santa Clara CA
Initial Cost
Land	25,767
Building and Improvements	407
Costs Capitalized Subsequent to Acquisition	1,440
Gross Amount Carried As of Year End
Building and Improvements	27,614
Total	27,614
Accumulated Depreciation	321
VA1 Reston VA [Member]
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	95,345
Gross Amount Carried As of Year End
Land	6,779
Building and Improvements	128,408
Total	135,187
Accumulated Depreciation	81,567
VA2 Reston VA [Member]
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	135,290
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	134,948
Total	140,487
Accumulated Depreciation	41,563
VA3 Reston VA
Initial Cost
Land	8,529
Building and Improvements	26,411
Costs Capitalized Subsequent to Acquisition	155,722
Gross Amount Carried As of Year End
Land	3,866
Building and Improvements	186,796
Total	190,662
Accumulated Depreciation	5,773
Reston Campus Expansion
Initial Cost
Land	5,671
Building and Improvements	17,538
Costs Capitalized Subsequent to Acquisition	4
Gross Amount Carried As of Year End
Land	5,672
Building and Improvements	17,541
Total	23,213
Accumulated Depreciation	$ 1,479