Consolidated Statements of Income - USD ($) shares in Thousands, $ 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
Revenues:
Rental revenue													$ 483,944	$ 460,653	$ 447,784
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
Operating expenses:
Property operating expenses													163,531	153,850	158,638
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
Interest 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
Private perpetual preferred unit distributions													(936)	(936)	(936)
Net income attributable to non-controlling interests													(54,670)	(54,858)	(45,262)
Net income attributable to common stockholders													$ 62,647	$ 51,456	$ 33,730
Total weighted average shares:
Basic (in shares)													158,380	133,881	114,245
Diluted (in shares)													298,049	277,568	266,621
Net income per share attributable to common stockholders:
Basic (USD per share)													$ 0.40	$ 0.38	$ 0.30
Diluted (USD per share)													$ 0.39	$ 0.38	$ 0.29