Segment Reporting - Segment Revenue and Profit or Loss Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Rental revenue	$ 112,613	$ 112,866	$ 227,521	$ 222,924
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	19,054	18,582	37,174	36,782
Observatory revenue	31,838	30,600	53,019	48,823
Construction revenue	0	374	0	1,981
Third-party management and other fees	423	594	968	1,040
Other revenue and fees	1,887	1,757	4,207	5,105
Total revenues	165,815	164,773	322,889	316,655
Operating expenses:
Property operating expenses	37,386	37,262	76,490	79,289
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,330	2,332	4,663	4,663
General and administrative expenses	12,907	9,113	23,825	18,213
Observatory expenses	6,895	8,093	14,650	15,495
Construction expenses	0	353	0	3,222
Real estate taxes	23,557	22,952	47,082	45,930
Acquisition expenses	0	0	98	0
Depreciation and amortization	38,548	39,629	77,775	81,047
Total operating expenses	121,623	119,734	244,583	247,859
Total operating income (loss)	44,192	45,039	78,306	68,796
Interest expense	(17,420)	(17,571)	(35,371)	(33,618)
Income before income taxes	26,772	27,468	42,935	35,178
Income tax expense	(2,132)	(883)	(1,590)	(705)
Net income (loss)	24,640	26,585	41,345	34,473
Segment assets	3,301,654	3,293,964	3,301,654	3,293,964	$ 3,300,650
Expenditures for segment assets	63,813	31,820	87,271	69,213
Operating segments | Real Estate
Segment Reporting Information [Line Items]
Rental revenue	112,613	112,866	227,521	222,924
Intercompany rental revenue	18,488	18,910	32,206	30,574
Tenant expense reimbursement	19,054	18,582	37,174	36,782
Observatory revenue	0	0	0	0
Construction revenue		0		0
Third-party management and other fees	423	594	968	1,040
Other revenue and fees	1,882	1,757	4,202	5,105
Total revenues	152,460	152,709	302,071	296,425
Operating expenses:
Property operating expenses	37,386	37,262	76,490	79,289
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,330	2,332	4,663	4,663
General and administrative expenses	12,907	9,113	23,825	18,213
Observatory expenses	0	0	0	0
Construction expenses		0		0
Real estate taxes	23,557	22,952	47,082	45,930
Acquisition expenses	0		98
Depreciation and amortization	38,451	39,542	77,581	80,775
Total operating expenses	114,631	111,201	229,739	228,870
Total operating income (loss)	37,829	41,508	72,332	67,555
Interest expense	(17,420)	(17,571)	(35,371)	(33,618)
Income before income taxes	20,409	23,937	36,961	33,937
Income tax expense	(281)	(422)	(686)	(773)
Net income (loss)	20,128	23,515	36,275	33,164
Segment assets	3,056,928	3,051,768	3,056,928	3,051,768
Expenditures for segment assets	63,766	31,796	87,224	69,189
Operating segments | Observatory
Segment Reporting Information [Line Items]
Rental revenue	0	0	0	0
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	0	0	0	0
Observatory revenue	31,838	30,600	53,019	48,823
Construction revenue		0		0
Third-party management and other fees	0	0	0	0
Other revenue and fees	5	0	5	0
Total revenues	31,843	30,600	53,024	48,823
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	18,488	18,910	32,206	30,574
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	6,895	8,093	14,650	15,495
Construction expenses		0		0
Real estate taxes	0	0	0	0
Acquisition expenses	0		0
Depreciation and amortization	97	87	194	171
Total operating expenses	25,480	27,090	47,050	46,240
Total operating income (loss)	6,363	3,510	5,974	2,583
Interest expense	0	0	0	0
Income before income taxes	6,363	3,510	5,974	2,583
Income tax expense	(1,851)	(528)	(904)	(272)
Net income (loss)	4,512	2,982	5,070	2,311
Segment assets	244,726	240,374	244,726	240,374
Expenditures for segment assets	47	24	47	24
Operating segments | Other
Segment Reporting Information [Line Items]
Rental revenue		0		0
Intercompany rental revenue		0		0
Tenant expense reimbursement		0		0
Observatory revenue		0		0
Construction revenue		1,774		5,696
Third-party management and other fees		0		0
Other revenue and fees		0		0
Total revenues		1,774		5,696
Operating expenses:
Property operating expenses		0		0
Intercompany rent expense		0		0
Ground rent expenses		0		0
General and administrative expenses		0		0
Observatory expenses		0		0
Construction expenses		1,566		6,539
Real estate taxes		0		0
Depreciation and amortization		0		101
Total operating expenses		1,566		6,640
Total operating income (loss)		208		(944)
Interest expense		0		0
Income before income taxes		208		(944)
Income tax expense		67		340
Net income (loss)		275		(604)
Segment assets		1,822		1,822
Expenditures for segment assets		0		0
Intersegment Elimination
Segment Reporting Information [Line Items]
Rental revenue	0	0	0	0
Intercompany rental revenue	(18,488)	(18,910)	(32,206)	(30,574)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Construction revenue		(1,400)		(3,715)
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(18,488)	(20,310)	(32,206)	(34,289)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(18,488)	(18,910)	(32,206)	(30,574)
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Construction expenses		(1,213)		(3,317)
Real estate taxes	0	0	0	0
Acquisition expenses	0		0
Depreciation and amortization	0	0	0	0
Total operating expenses	(18,488)	(20,123)	(32,206)	(33,891)
Total operating income (loss)	0	(187)	0	(398)
Interest expense	0	0	0	0
Income before income taxes	0	(187)	0	(398)
Income tax expense	0	0	0	0
Net income (loss)	0	(187)	0	(398)
Segment assets	0	0	0	0
Expenditures for segment assets	$ 0	$ 0	$ 0	$ 0