Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Initial Cost
Land	$ 103,889
Building and Improvements	424,905
Costs Capitalized Subsequent to Acquisition	1,114,782
Gross Amount Carried As of Year End
Land	100,258
Building and Improvements	1,543,318
Total	1,643,576	$ 1,295,135	$ 1,146,548	$ 1,048,525
Accumulated Depreciation	369,303	$ 284,219	$ 215,978	$ 155,704
Aggregate cost of the total properties for federal income tax purposes	1,592,600
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	68,943
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	96,637
Total	101,791
Accumulated Depreciation	43,636
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	56,227
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	105,749
Total	111,242
Accumulated Depreciation	28,918
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	3,851
Gross Amount Carried As of Year End
Building and Improvements	8,648
Total	8,648
Accumulated Depreciation	5,237
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	9,972
Gross Amount Carried As of Year End
Building and Improvements	10,009
Total	10,009
Accumulated Depreciation	1,496
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	1,319
Gross Amount Carried As of Year End
Building and Improvements	1,323
Total	1,323
Accumulated Depreciation	797
LA1 Los Angeles CA
Initial Cost
Building and Improvements	41,099
Costs Capitalized Subsequent to Acquisition	33,947
Gross Amount Carried As of Year End
Building and Improvements	75,046
Total	75,046
Accumulated Depreciation	33,584
LA2 Los Angeles CA
Initial Cost
Land	28,467
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	71,900
Gross Amount Carried As of Year End
Land	28,467
Building and Improvements	166,014
Total	194,481
Accumulated Depreciation	42,653
MI1 Miami FL
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	1,174
Gross Amount Carried As of Year End
Land	728
Building and Improvements	10,499
Total	11,227
Accumulated Depreciation	2,771
NY1 New York NY
Initial Cost
Costs Capitalized Subsequent to Acquisition	35,581
Gross Amount Carried As of Year End
Building and Improvements	35,581
Total	35,581
Accumulated Depreciation	19,445
NY2 Secaucus NJ
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	110,157
Gross Amount Carried As of Year End
Land	2,388
Building and Improvements	131,129
Total	133,517
Accumulated Depreciation	14,761
SV1 San Jose CA
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	39,902
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	131,621
Total	138,484
Accumulated Depreciation	29,694
SV2 Milpitas CA
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	24,127
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	29,173
Total	34,259
Accumulated Depreciation	17,071
SV3 Santa Clara CA
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	47,939
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	51,611
Total	55,583
Accumulated Depreciation	29,811
SV4 Santa Clara CA
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	95,088
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	98,935
Total	103,436
Accumulated Depreciation	30,159
SV5 Santa Clara CA
Initial Cost
Land	2,572
Costs Capitalized Subsequent to Acquisition	23,613
Gross Amount Carried As of Year End
Land	2,544
Building and Improvements	23,641
Total	26,185
Accumulated Depreciation	1,895
SV6 Santa Clara CA
Initial Cost
Land	4,741
Costs Capitalized Subsequent to Acquisition	36,852
Gross Amount Carried As of Year End
Land	4,825
Building and Improvements	36,768
Total	41,593
Accumulated Depreciation	615
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	235,126
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	235,324
Total	238,919
Accumulated Depreciation	2,606
VA1 Reston VA
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	89,705
Gross Amount Carried As of Year End
Land	6,903
Building and Improvements	122,644
Total	129,547
Accumulated Depreciation	52,542
VA2 Reston VA
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	128,169
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	127,827
Total	133,366
Accumulated Depreciation	11,444
Reston Campus Expansion
Initial Cost
Land	14,200
Building and Improvements	43,949
Costs Capitalized Subsequent to Acquisition	1,190
Gross Amount Carried As of Year End
Land	14,200
Building and Improvements	45,139
Total	59,339
Accumulated Depreciation	$ 168