Segment Reporting - Components of Segment Profit for Each Segment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenues:
Rental revenue	$ 122,311	$ 117,113
Intercompany rental revenue	0	0
Tenant expense reimbursement	17,794	15,974
Observatory revenue	21,249	20,940
Lease termination fees	622	7,938
Third-party management and other fees	463	351
Other revenue and fees	6,057	2,638
Total revenues	168,496	164,954
Operating expenses:
Property operating expenses	44,185	42,210
Intercompany rent expense	0	0
Ground rent expenses	2,331	2,331
General and administrative expenses	12,628	11,088
Observatory expenses	7,336	7,255
Real estate taxes	26,744	24,558
Depreciation and amortization	39,883	40,846
Total operating expenses	133,107	128,288
Total operating income	35,389	36,666
Other expenses:
Interest expense	(17,591)	(17,742)
Loss from derivative financial instruments	0	(247)
Income before income taxes	17,798	18,677
Income tax (expense) benefit	260	468
Net income	18,058	19,145
Segment assets	4,141,813	3,866,176	$ 3,931,347
Expenditures for segment assets	46,350	48,060
Intersegment Elimination
Revenues:
Rental revenue	0	0
Intercompany rental revenue	(13,561)	(13,378)
Tenant expense reimbursement	0	0
Observatory revenue	0	0
Lease termination fees	0	0
Third-party management and other fees	0	0
Other revenue and fees	0	0
Total revenues	(13,561)	(13,378)
Operating expenses:
Property operating expenses	0	0
Intercompany rent expense	(13,561)	(13,378)
Ground rent expenses	0	0
General and administrative expenses	0	0
Observatory expenses	0	0
Real estate taxes	0	0
Depreciation and amortization	0	0
Total operating expenses	(13,561)	(13,378)
Total operating income	0	0
Other expenses:
Interest expense	0	0
Loss from derivative financial instruments		0
Income before income taxes	0	0
Income tax (expense) benefit	0	0
Net income	0	0
Segment assets	0	0
Expenditures for segment assets	0	0
Real Estate | Operating Segments
Revenues:
Rental revenue	122,311	117,113
Intercompany rental revenue	13,561	13,378
Tenant expense reimbursement	17,794	15,974
Observatory revenue	0	0
Lease termination fees	622	7,938
Third-party management and other fees	463	351
Other revenue and fees	6,057	2,638
Total revenues	160,808	157,392
Operating expenses:
Property operating expenses	44,185	42,210
Intercompany rent expense	0	0
Ground rent expenses	2,331	2,331
General and administrative expenses	12,628	11,088
Observatory expenses	0	0
Real estate taxes	26,744	24,558
Depreciation and amortization	39,865	40,833
Total operating expenses	125,753	121,020
Total operating income	35,055	36,372
Other expenses:
Interest expense	(17,591)	(17,742)
Loss from derivative financial instruments		(247)
Income before income taxes	17,464	18,383
Income tax (expense) benefit	(307)	(283)
Net income	17,157	18,100
Segment assets	3,887,337	3,617,276
Expenditures for segment assets	34,797	48,054
Observatory | Operating Segments
Revenues:
Rental revenue	0	0
Intercompany rental revenue	0	0
Tenant expense reimbursement	0	0
Observatory revenue	21,249	20,940
Lease termination fees	0	0
Third-party management and other fees	0	0
Other revenue and fees	0	0
Total revenues	21,249	20,940
Operating expenses:
Property operating expenses	0	0
Intercompany rent expense	13,561	13,378
Ground rent expenses	0	0
General and administrative expenses	0	0
Observatory expenses	7,336	7,255
Real estate taxes	0	0
Depreciation and amortization	18	13
Total operating expenses	20,915	20,646
Total operating income	334	294
Other expenses:
Interest expense	0	0
Loss from derivative financial instruments		0
Income before income taxes	334	294
Income tax (expense) benefit	567	751
Net income	901	1,045
Segment assets	254,476	248,900
Expenditures for segment assets	$ 11,553	$ 6