Segment Reporting - Segment Revenue and Profit or Loss Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													$ 586,414
Rental revenue														$ 493,231	$ 483,944
Intercompany rental revenue													0	0	0
Tenant expense reimbursement													0	72,372	73,679
Observatory revenue													128,769	131,227	127,118
Lease termination fees													4,352	20,847	13,551
Third-party management and other fees													1,254	1,440	1,400
Other revenue and fees													10,554	12,394	9,834
Total revenues	$ 194,933	$ 192,873	$ 176,244	$ 167,293	$ 199,309	$ 186,402	$ 178,529	$ 167,271	$ 182,297	$ 186,547	$ 176,349	$ 164,333	731,343	731,511	709,526
Property operating expenses													174,977	167,379	163,531
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													61,063	52,674	50,315
Observatory expenses													33,767	32,767	30,275
Real estate taxes													115,916	110,000	102,466
Acquisition expenses															0
Depreciation and amortization													181,588	168,508	160,710
Total operating expenses													576,637	540,654	516,623
Total operating income	47,112	45,279	36,239	26,076	58,490	48,538	49,665	34,164	50,191	56,008	50,659	36,045	154,706	190,857	192,903
Interest income													11,259	10,661	2,942
Interest expense													(79,246)	(79,623)	(68,473)
Loss on early extinguishment of debt													0	0	(2,157)
Loss from derivative financial instruments													0	0	(289)
Income before income taxes													86,719	121,895	124,926
Income tax expense													(2,429)	(4,642)	(6,673)
Net income	28,720	$ 26,784	$ 18,930	$ 9,856	39,781	$ 29,230	$ 30,184	$ 18,058	32,260	$ 35,489	$ 31,359	$ 19,145	84,290	117,253	118,253
Segment assets	3,931,834				4,195,780				3,931,347				3,931,834	4,195,780	3,931,347
Expenditures for segment assets													255,924	256,496	228,162
Intersegment Elimination
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0
Rental revenue														0	0
Intercompany rental revenue													(82,469)	(79,954)	(77,646)
Tenant expense reimbursement														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													(82,469)	(79,954)	(77,646)
Property operating expenses													0	0	0
Intercompany rent expense													(82,469)	(79,954)	(77,646)
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													(82,469)	(79,954)	(77,646)
Total operating income													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	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													586,414
Rental revenue														493,231	483,944
Intercompany rental revenue													82,469	79,954	77,646
Tenant expense reimbursement														72,372	73,679
Observatory revenue													0	0	0
Lease termination fees													4,352	20,847	13,551
Third-party management and other fees													1,254	1,440	1,400
Other revenue and fees													10,554	12,394	9,834
Total revenues													685,043	680,238	660,054
Property operating expenses													174,977	167,379	163,531
Intercompany rent expense													0	0	0
Ground rent expenses													9,326	9,326	9,326
General and administrative expenses													61,063	52,674	50,315
Observatory expenses													0	0	0
Real estate taxes													115,916	110,000	102,466
Acquisition expenses															0
Depreciation and amortization													181,558	168,430	160,630
Total operating expenses													542,840	507,809	486,268
Total operating income													142,203	172,429	173,786
Interest income													11,259	10,661	2,942
Interest expense													(79,246)	(79,623)	(68,473)
Loss on early extinguishment of debt														0	(2,157)
Loss from derivative financial instruments														0	(289)
Income before income taxes													74,216	103,467	105,809
Income tax expense													(896)	(1,114)	(1,306)
Net income													73,320	102,353	104,503
Segment assets	3,671,211				3,930,330				3,670,907				3,671,211	3,930,330	3,670,907
Expenditures for segment assets													191,630	201,685	191,541
Observatory | Operating Segments
Reconciliation of Revenue and Profit or Loss from Segments to Consolidated [Line Items]
Rental revenue													0
Rental revenue														0	0
Intercompany rental revenue													0	0	0
Tenant expense reimbursement														0	0
Observatory revenue													128,769	131,227	127,118
Lease termination fees													0	0	0
Third-party management and other fees													0	0	0
Other revenue and fees													0	0	0
Total revenues													128,769	131,227	127,118
Property operating expenses													0	0	0
Intercompany rent expense													82,469	79,954	77,646
Ground rent expenses													0	0	0
General and administrative expenses													0	0	0
Observatory expenses													33,767	32,767	30,275
Real estate taxes													0	0	0
Acquisition expenses															0
Depreciation and amortization													30	78	80
Total operating expenses													116,266	112,799	108,001
Total operating income													12,503	18,428	19,117
Interest income													0	0	0
Interest expense													0	0	0
Loss on early extinguishment of debt														0	0
Loss from derivative financial instruments														0	0
Income before income taxes													12,503	18,428	19,117
Income tax expense													(1,533)	(3,528)	(5,367)
Net income													10,970	14,900	13,750
Segment assets	$ 260,623				$ 265,450				$ 260,440				260,623	265,450	260,440
Expenditures for segment assets													$ 64,294	$ 54,811	$ 36,621