Consolidated Statements of Income - USD ($) shares in Thousands, $ 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
Revenues:
Rental revenue													$ 493,231	$ 483,944	$ 460,653
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
Operating expenses:
Property operating expenses													167,379	163,531	153,850
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
Other income (expense):
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
Private perpetual preferred unit distributions													(936)	(936)	(936)
Net income attributable to non-controlling interests													(50,714)	(54,670)	(54,858)
Net income attributable to common stockholders													$ 65,603	$ 62,647	$ 51,456
Total weighted average shares:
Basic (in shares)													167,571	158,380	133,881
Diluted (in shares)													297,259	298,049	277,568
Earnings per share attributable to common stockholders:
Basic (USD per share)													$ 0.39	$ 0.40	$ 0.38
Diluted (USD per share)													$ 0.39	$ 0.39	$ 0.38