Segment Reporting - Segment Revenue and Profit or Loss Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													$ 483,944	$ 460,653	$ 447,784
Intercompany rental revenue													0	0	0
Tenant expense reimbursement													73,679	73,459	79,516
Observatory revenue													127,118	124,814	112,172
Construction revenue													0	0	1,981
Third-party management and other fees													1,400	1,766	2,133
Other revenue and fees													26,327	17,308	14,048
Total revenues	$ 183,070	$ 187,320	$ 177,124	$ 164,954	$ 179,263	$ 175,848	$ 165,815	$ 157,074	$ 165,200	$ 175,779	$ 164,773	$ 151,882	712,468	678,000	657,634
Property operating expenses													163,531	153,850	158,638
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													50,315	49,078	38,073
Observatory expenses													30,275	29,833	32,174
Construction expenses													0	0	3,222
Real estate taxes													102,466	96,061	93,165
Acquisition expenses													0	98	193
Depreciation and amortization													160,710	155,211	171,474
Total operating expenses													516,623	493,457	506,265
Total operating income (loss)	50,964	56,781	51,434	36,666	52,651	53,586	44,192	34,114	37,230	45,343	45,039	23,757	195,845	184,543	151,369
Income tax expense													(68,473)	(70,595)	(65,743)
Loss on early extinguishment of debt													(2,157)	(552)	(1,749)
Loss from derivative financial instruments													(289)	0	0
Income before income taxes													124,926	113,396	83,877
Income tax expense													(6,673)	(6,146)	(3,949)
Net income	32,260	$ 35,489	$ 31,359	$ 19,145	33,008	$ 32,897	$ 24,640	$ 16,705	19,370	$ 26,085	$ 26,585	$ 7,888	118,253	107,250	79,928
Segment assets	3,931,347				3,890,953				3,300,650				3,931,347	3,890,953	3,300,650
Expenditures for segment assets													228,162	197,680	156,754
Intersegment Eliminations [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0	0	0
Intercompany rental revenue													(77,646)	(75,658)	(68,255)
Tenant expense reimbursement													0	0	0
Observatory revenue													0	0	0
Construction revenue															(3,715)
Third-party management and other fees													0	0	0
Other revenue and fees													0	0	0
Total revenues													(77,646)	(75,658)	(71,970)
Property operating expenses													0	0	0
Intercompany rent expense													(77,646)	(75,658)	(68,255)
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													0	0	0
Construction expenses															(3,317)
Real estate taxes													0	0	0
Acquisition expenses														0	0
Depreciation and amortization													0	0	0
Total operating expenses													(77,646)	(75,658)	(71,572)
Total operating income (loss)													0	0	(398)
Income tax expense													0	0	0
Loss on early extinguishment of debt													0	0
Loss from derivative financial instruments													0
Income before income taxes													0	0	(398)
Income tax expense													0	0	0
Net income													0	0	(398)
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													483,944	460,653	447,784
Intercompany rental revenue													77,646	75,658	68,255
Tenant expense reimbursement													73,679	73,459	79,516
Observatory revenue													0	0	0
Construction revenue															0
Third-party management and other fees													1,400	1,766	2,133
Other revenue and fees													26,327	17,293	14,048
Total revenues													662,996	628,829	611,736
Property operating expenses													163,531	153,850	158,638
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													50,315	49,078	38,073
Observatory expenses													0	0	0
Construction expenses															0
Real estate taxes													102,466	96,061	93,165
Acquisition expenses														98	193
Depreciation and amortization													160,630	154,817	171,035
Total operating expenses													486,268	463,230	470,430
Total operating income (loss)													176,728	165,599	141,306
Income tax expense													(68,473)	(70,595)	(65,743)
Loss on early extinguishment of debt													(2,157)	(552)	(1,749)
Loss from derivative financial instruments													(289)
Income before income taxes													105,809	94,452	73,814
Income tax expense													(1,306)	(1,361)	(1,498)
Net income													104,503	93,091	72,316
Segment assets	3,670,907				3,641,844				3,058,250				3,670,907	3,641,844	3,058,250
Expenditures for segment assets													191,541	197,680	156,543
Observatory, Segment [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													127,118	124,814	112,172
Construction revenue															0
Third-party management and other fees													0	0	0
Other revenue and fees													0	15	0
Total revenues													127,118	124,829	112,172
Property operating expenses													0	0	0
Intercompany rent expense													77,646	75,658	68,255
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													30,275	29,833	32,174
Construction expenses															0
Real estate taxes													0	0	0
Acquisition expenses														0	0
Depreciation and amortization													80	394	338
Total operating expenses													108,001	105,885	100,767
Total operating income (loss)													19,117	18,944	11,405
Income tax expense													0	0	0
Loss on early extinguishment of debt													0	0
Loss from derivative financial instruments													0
Income before income taxes													19,117	18,944	11,405
Income tax expense													(5,367)	(4,785)	(2,791)
Net income													13,750	14,159	8,614
Segment assets	$ 260,440				$ 249,109				241,511				260,440	249,109	241,511
Expenditures for segment assets													$ 36,621	$ 0	211
Other [Member] | Operating Segments [Member]
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue															0
Intercompany rental revenue															0
Tenant expense reimbursement															0
Observatory revenue															0
Construction revenue															5,696
Third-party management and other fees															0
Other revenue and fees															0
Total revenues															5,696
Property operating expenses															0
Intercompany rent expense															0
Ground rent expenses															0
General and administrative expenses															0
Observatory expenses															0
Construction expenses															6,539
Real estate taxes															0
Acquisition expenses															0
Depreciation and amortization															101
Total operating expenses															6,640
Total operating income (loss)															(944)
Income tax expense															0
Income before income taxes															(944)
Income tax expense															340
Net income															(604)
Segment assets									$ 889						889
Expenditures for segment assets															$ 0