Segment Reporting - Segment Revenue and Profit or Loss Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													$ 460,653	$ 447,784	$ 400,825
Intercompany rental revenue													0	0	0
Tenant expense reimbursement													73,459	79,516	67,651
Observatory revenue													124,814	112,172	111,541
Construction revenue													0	1,981	38,648
Third-party management and other fees													1,766	2,133	2,376
Other revenue and fees													17,308	14,048	14,285
Total revenues	$ 179,263	$ 175,848	$ 165,815	$ 157,074	$ 165,200	$ 175,779	$ 164,773	$ 151,882	$ 170,411	$ 169,441	$ 155,168	$ 140,306	678,000	657,634	635,326
Property operating expenses													153,850	158,638	148,676
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	5,339
General and administrative expenses													49,078	38,073	39,037
Observatory expenses													29,833	32,174	31,413
Construction expenses													0	3,222	38,596
Real estate taxes													96,061	93,165	82,131
Acquisition expenses													98	193	3,382
Depreciation and amortization													155,211	171,474	145,431
Total operating expenses													493,457	506,265	494,005
Total operating income (loss)	52,651	53,586	44,192	34,114	37,230	45,343	45,039	23,757	31,282	43,412	42,539	24,088	184,543	151,369	141,321
Income tax (expense) benefit													(71,147)	(67,492)	(66,456)
Income before income taxes													113,396	83,877	74,865
Income tax expense													(6,146)	(3,949)	(4,655)
Net income	33,008	$ 32,897	$ 24,640	$ 16,705	19,370	$ 26,085	$ 26,585	$ 7,888	10,964	$ 22,734	$ 25,281	$ 11,231	107,250	79,928	70,210
Segment assets	3,890,953				3,300,650				3,283,497				3,890,953	3,300,650	3,283,497
Expenditures for segment assets													197,680	156,754	497,744
Intersegment Eliminations [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0	0	0
Intercompany rental revenue													(75,658)	(68,255)	(69,293)
Tenant expense reimbursement													0	0	0
Observatory revenue													0	0	0
Construction revenue														(3,715)	(6,341)
Third-party management and other fees													0	0	0
Other revenue and fees													0	0	0
Total revenues													(75,658)	(71,970)	(75,634)
Property operating expenses													0	0	0
Intercompany rent expense													(75,658)	(68,255)	(69,293)
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													0	0	0
Construction expenses														(3,317)	(5,589)
Real estate taxes													0	0	0
Acquisition expenses													0	0	0
Depreciation and amortization													0	0	0
Total operating expenses													(75,658)	(71,572)	(74,882)
Total operating income (loss)													0	(398)	(752)
Income tax (expense) benefit													0	0	0
Income before income taxes													0	(398)	(752)
Income tax expense													0	0	0
Net income													0	(398)	(752)
Segment assets	0				0				0				0	0	0
Expenditures for segment assets													0	0	0
Real Estate [Member] | Operating Segments [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													460,653	447,784	400,825
Intercompany rental revenue													75,658	68,255	69,293
Tenant expense reimbursement													73,459	79,516	67,651
Observatory revenue													0	0	0
Construction revenue														0	0
Third-party management and other fees													1,766	2,133	2,376
Other revenue and fees													17,293	14,048	13,280
Total revenues													628,829	611,736	553,425
Property operating expenses													153,850	158,638	148,676
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	5,339
General and administrative expenses													49,078	38,073	39,037
Observatory expenses													0	0	0
Construction expenses														0	0
Real estate taxes													96,061	93,165	82,131
Acquisition expenses													98	193	3,382
Depreciation and amortization													154,817	171,035	145,121
Total operating expenses													463,230	470,430	423,686
Total operating income (loss)													165,599	141,306	129,739
Income tax (expense) benefit													(71,147)	(67,492)	(66,456)
Income before income taxes													94,452	73,814	63,283
Income tax expense													(1,361)	(1,498)	(1,437)
Net income													93,091	72,316	61,846
Segment assets	3,641,844				3,058,250				3,023,089				3,641,844	3,058,250	3,023,089
Expenditures for segment assets													197,680	156,543	497,645
Observatory [Member] | Operating Segments [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0	0	0
Intercompany rental revenue													0	0	0
Tenant expense reimbursement													0	0	0
Observatory revenue													124,814	112,172	111,541
Construction revenue														0	0
Third-party management and other fees													0	0	0
Other revenue and fees													15	0	982
Total revenues													124,829	112,172	112,523
Property operating expenses													0	0	0
Intercompany rent expense													75,658	68,255	69,293
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													29,833	32,174	31,413
Construction expenses														0	0
Real estate taxes													0	0	0
Acquisition expenses													0	0	0
Depreciation and amortization													394	338	295
Total operating expenses													105,885	100,767	101,001
Total operating income (loss)													18,944	11,405	11,522
Income tax (expense) benefit													0	0	0
Income before income taxes													18,944	11,405	11,522
Income tax expense													(4,785)	(2,791)	(2,876)
Net income													14,159	8,614	8,646
Segment assets	$ 249,109				241,511				252,376				249,109	241,511	252,376
Expenditures for segment assets													$ 0	211	99
Other [Member] | Operating Segments [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue														0	0
Intercompany rental revenue														0	0
Tenant expense reimbursement														0	0
Observatory revenue														0	0
Construction revenue														5,696	44,989
Third-party management and other fees														0	0
Other revenue and fees														0	23
Total revenues														5,696	45,012
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														6,539	44,185
Real estate taxes														0	0
Acquisition expenses														0	0
Depreciation and amortization														101	15
Total operating expenses														6,640	44,200
Total operating income (loss)														(944)	812
Income tax (expense) benefit														0	0
Income before income taxes														(944)	812
Income tax expense														340	(342)
Net income														(604)	470
Segment assets					$ 889				$ 8,032					889	8,032
Expenditures for segment assets														$ 0	$ 0