Deferred Costs, Acquired Lease Intangibles and Goodwill - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Deferred Costs [Line Items]
Amortization expense related to deferred leasing costs	$ 6,300	$ 5,800	$ 18,400	$ 17,900
Revenue related to amortization of acquired above- and below-market leases, net	1,700	1,600	4,387	4,154
Goodwill	491,479		491,479		$ 491,479
Observatory
Deferred Costs [Line Items]
Goodwill	227,500		227,500
Real estate
Deferred Costs [Line Items]
Goodwill	264,000		264,000
Lease agreements
Deferred Costs [Line Items]
Amortization expense related to acquired lease intangibles	2,900	$ 4,000	9,300	$ 13,000
Unsecured revolving credit facility | Unsecured term loan facility
Deferred Costs [Line Items]
Net deferred financing costs	$ 6,800		$ 6,800		$ 8,300