Segment Reporting - Components of Segment Profit for Each Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Revenues:
Rental revenue	$ 120,844	$ 112,613	$ 237,957	$ 227,521
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	17,569	19,054	33,543	37,174
Observatory revenue	33,966	31,838	54,906	53,019
Third-party management and other fees	392	423	743	968
Other revenue and fees	4,353	1,887	14,929	4,207
Total revenues	177,124	165,815	342,078	322,889
Operating expenses:
Property operating expenses	38,529	37,386	80,739	76,490
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	2,330	4,663	4,663
General and administrative expenses	12,579	12,907	23,667	23,825
Observatory expenses	7,176	6,895	14,431	14,650
Real estate taxes	24,542	23,557	49,100	47,082
Acquisition expenses	0	0	0	98
Depreciation and amortization	40,532	38,548	81,378	77,775
Total operating expenses	125,690	121,623	253,978	244,583
Total operating income	51,434	44,192	88,100	78,306
Other expense:
Interest expense	(17,477)	(17,420)	(35,219)	(35,371)
Loss from derivative financial instruments	(42)	0	(289)	0
Income before income taxes	33,915	26,772	52,592	42,935
Income tax expense	(2,556)	(2,132)	(2,088)	(1,590)
Net income	31,359	24,640	50,504	41,345
Segment assets	3,831,201	3,301,654	3,831,201	3,301,654	$ 3,890,953
Expenditures for segment assets	61,324	63,813	109,384	87,271
Intersegment Elimination
Revenues:
Rental revenue	0	0	0	0
Intercompany rental revenue	(20,675)	(18,488)	(34,053)	(32,206)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(20,675)	(18,488)	(34,053)	(32,206)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(20,675)	(18,488)	(34,053)	(32,206)
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Real estate taxes	0	0	0	0
Acquisition expenses		0	0	0
Depreciation and amortization	0	0	0	0
Total operating expenses	(20,675)	(18,488)	(34,053)	(32,206)
Total operating income	0	0	0	0
Other expense:
Interest expense	0	0	0	0
Loss from derivative financial instruments	0		0
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Segment assets	0	0	0	0
Expenditures for segment assets	0	0	0	0
Real Estate | Operating Segments
Revenues:
Rental revenue	120,844	112,613	237,957	227,521
Intercompany rental revenue	20,675	18,488	34,053	32,206
Tenant expense reimbursement	17,569	19,054	33,543	37,174
Observatory revenue	0	0	0	0
Third-party management and other fees	392	423	743	968
Other revenue and fees	4,353	1,882	14,929	4,202
Total revenues	163,833	152,460	321,225	302,071
Operating expenses:
Property operating expenses	38,529	37,386	80,739	76,490
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	2,330	4,663	4,663
General and administrative expenses	12,579	12,907	23,667	23,825
Observatory expenses	0	0	0	0
Real estate taxes	24,542	23,557	49,100	47,082
Acquisition expenses		0	0	98
Depreciation and amortization	40,458	38,451	81,291	77,581
Total operating expenses	118,440	114,631	239,460	229,739
Total operating income	45,393	37,829	81,765	72,332
Other expense:
Interest expense	(17,477)	(17,420)	(35,219)	(35,371)
Loss from derivative financial instruments	(42)		(289)
Income before income taxes	27,874	20,409	46,257	36,961
Income tax expense	(413)	(281)	(696)	(686)
Net income	27,461	20,128	45,561	36,275
Segment assets	3,579,873	3,056,928	3,579,873	3,056,928
Expenditures for segment assets	54,474	63,766	102,534	87,224
Observatory | Operating Segments
Revenues:
Rental revenue	0	0	0	0
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	0	0	0	0
Observatory revenue	33,966	31,838	54,906	53,019
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	5	0	5
Total revenues	33,966	31,843	54,906	53,024
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	20,675	18,488	34,053	32,206
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	7,176	6,895	14,431	14,650
Real estate taxes	0	0	0	0
Acquisition expenses		0	0	0
Depreciation and amortization	74	97	87	194
Total operating expenses	27,925	25,480	48,571	47,050
Total operating income	6,041	6,363	6,335	5,974
Other expense:
Interest expense	0	0	0	0
Loss from derivative financial instruments	0		0
Income before income taxes	6,041	6,363	6,335	5,974
Income tax expense	(2,143)	(1,851)	(1,392)	(904)
Net income	3,898	4,512	4,943	5,070
Segment assets	251,328	244,726	251,328	244,726
Expenditures for segment assets	$ 6,850	$ 47	$ 6,850	$ 47