Segment Reporting - Segment Revenue and Profit or Loss Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													$ 493,231	$ 483,944	$ 460,653
Intercompany rental revenue													0	0	0
Tenant expense reimbursement													72,372	73,679	73,459
Observatory revenue													131,227	127,118	124,814
Lease termination fees													20,847	13,551	7,676
Third-party management and other fees													1,440	1,400	1,766
Other revenue and fees													12,394	9,834	8,985
Total revenues	$ 199,309	$ 186,402	$ 178,529	$ 167,271	$ 182,297	$ 186,547	$ 176,349	$ 164,333	$ 178,807	$ 175,704	$ 165,785	$ 157,057	731,511	709,526	677,353
Property operating expenses													167,379	163,531	153,850
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													52,674	50,315	49,078
Observatory expenses													32,767	30,275	29,833
Real estate taxes													110,000	102,466	96,061
Acquisition expenses													0	0	98
Depreciation and amortization													168,508	160,710	155,211
Total operating expenses													540,654	516,623	493,457
Total operating income (loss)	58,490	48,538	49,665	34,164	50,191	56,008	50,659	36,045	52,195	53,442	44,162	34,097	190,857	192,903	183,896
Interest income													10,661	2,942	647
Interest expense													(79,623)	(68,473)	(70,595)
Loss on early extinguishment of debt													0	(2,157)	(552)
Loss from derivative financial instruments													0	(289)	0
Income before income taxes													121,895	124,926	113,396
Income tax expense													(4,642)	(6,673)	(6,146)
Net income	39,781	$ 29,230	$ 30,184	$ 18,058	32,260	$ 35,489	$ 31,359	$ 19,145	33,008	$ 32,897	$ 24,640	$ 16,705	117,253	118,253	107,250
Segment assets	4,195,780				3,931,347				3,890,953				4,195,780	3,931,347	3,890,953
Expenditures for segment assets													256,496	228,162	197,680
Intersegment Elimination
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0	0	0
Intercompany rental revenue													(79,954)	(77,646)	(75,658)
Tenant expense reimbursement													0	0	0
Observatory revenue													0	0	0
Lease termination fees													0	0	0
Third-party management and other fees													0	0	0
Other revenue and fees													0	0	0
Total revenues													(79,954)	(77,646)	(75,658)
Property operating expenses													0	0	0
Intercompany rent expense													(79,954)	(77,646)	(75,658)
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													0	0	0
Real estate taxes													0	0	0
Acquisition expenses															0
Depreciation and amortization													0	0	0
Total operating expenses													(79,954)	(77,646)	(75,658)
Total operating income (loss)													0	0	0
Interest income													0	0	0
Interest expense													0	0	0
Loss on early extinguishment of debt														0	0
Loss from derivative financial instruments														0
Income before income taxes													0	0	0
Income tax expense													0	0	0
Net income													0	0	0
Segment assets	0				0				0				0	0	0
Expenditures for segment assets													0	0	0
Real Estate | Operating Segments
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													493,231	483,944	460,653
Intercompany rental revenue													79,954	77,646	75,658
Tenant expense reimbursement													72,372	73,679	73,459
Observatory revenue													0	0	0
Lease termination fees													20,847	13,551	7,676
Third-party management and other fees													1,440	1,400	1,766
Other revenue and fees													12,394	9,834	8,970
Total revenues													680,238	660,054	628,182
Property operating expenses													167,379	163,531	153,850
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													52,674	50,315	49,078
Observatory expenses													0	0	0
Real estate taxes													110,000	102,466	96,061
Acquisition expenses															98
Depreciation and amortization													168,430	160,630	154,817
Total operating expenses													507,809	486,268	463,230
Total operating income (loss)													172,429	173,786	164,952
Interest income													10,661	2,942	647
Interest expense													(79,623)	(68,473)	(70,595)
Loss on early extinguishment of debt														(2,157)	(552)
Loss from derivative financial instruments														(289)
Income before income taxes													103,467	105,809	94,452
Income tax expense													(1,114)	(1,306)	(1,361)
Net income													102,353	104,503	93,091
Segment assets	3,930,330				3,670,907				3,641,844				3,930,330	3,670,907	3,641,844
Expenditures for segment assets													201,685	191,541	197,680
Observatory | Operating Segments
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													131,227	127,118	124,814
Lease termination fees													0	0	0
Third-party management and other fees													0	0	0
Other revenue and fees													0	0	15
Total revenues													131,227	127,118	124,829
Property operating expenses													0	0	0
Intercompany rent expense													79,954	77,646	75,658
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													32,767	30,275	29,833
Real estate taxes													0	0	0
Acquisition expenses															0
Depreciation and amortization													78	80	394
Total operating expenses													112,799	108,001	105,885
Total operating income (loss)													18,428	19,117	18,944
Interest income													0	0	0
Interest expense													0	0	0
Loss on early extinguishment of debt														0	0
Loss from derivative financial instruments														0
Income before income taxes													18,428	19,117	18,944
Income tax expense													(3,528)	(5,367)	(4,785)
Net income													14,900	13,750	14,159
Segment assets	$ 265,450				$ 260,440				$ 249,109				265,450	260,440	249,109
Expenditures for segment assets													$ 54,811	$ 36,621	$ 0