Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Line Items]
Capitalized internal leasing costs		$ 2,600
Minimum
Accounting Policies [Line Items]
Observatory revenue realized during the first quarter, previous ten years (as a percent)	16.00%
Observatory revenue realized during the second quarter, previous ten years (as a percent)	26.00%
Observatory revenue realized during the third quarter, previous ten years (as a percent)	31.00%
Observatory revenue realized during the fourth quarter, previous ten years (as a percent)	23.00%
Maximum
Accounting Policies [Line Items]
Observatory revenue realized during the first quarter, previous ten years (as a percent)	18.00%
Observatory revenue realized during the second quarter, previous ten years (as a percent)	28.00%
Observatory revenue realized during the third quarter, previous ten years (as a percent)	33.00%
Observatory revenue realized during the fourth quarter, previous ten years (as a percent)	25.00%
Ground lease agreements
Accounting Policies [Line Items]
Remaining contractual payments under ground lease agreements	$ 60,904