Segment Reporting - Components of Segment Profit for Each Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Revenues:
Rental revenue	$ 122,391	$ 115,634	$ 360,348	$ 343,155
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	20,346	19,176	53,889	56,350
Observatory revenue	39,306	38,093	94,212	91,112
Third-party management and other fees	345	404	1,088	1,372
Other revenue and fees	4,932	2,541	19,861	6,748
Total revenues	187,320	175,848	529,398	498,737
Operating expenses:
Property operating expenses	41,270	38,585	122,009	115,075
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,331	6,994	6,994
General and administrative expenses	12,899	11,798	36,566	35,623
Observatory expenses	8,648	7,250	23,079	21,900
Real estate taxes	26,901	24,691	76,001	71,773
Acquisition expenses	0	0	0	98
Depreciation and amortization	38,490	37,607	119,868	115,382
Total operating expenses	130,539	122,262	384,517	366,845
Total operating income	56,781	53,586	144,881	131,892
Other expenses:
Interest expense	(16,890)	(17,939)	(52,109)	(52,758)
Loss on early extinguishment of debt	(2,157)	0	(2,157)	(552)
Loss from derivative financial instruments	0	0	(289)	0
Income before income taxes	37,734	35,647	90,326	78,582
Income tax expense	(2,245)	(2,750)	(4,333)	(4,340)
Net income	35,489	32,897	85,993	74,242
Segment assets	3,848,934	3,882,569	3,848,934	3,882,569	$ 3,890,953
Expenditures for segment assets	53,731	52,828	163,115	140,099
Intersegment Elimination
Revenues:
Rental revenue	0	0	0	0
Intercompany rental revenue	(23,301)	(22,983)	(57,354)	(55,189)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(23,301)	(22,983)	(57,354)	(55,189)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(23,301)	(22,983)	(57,354)	(55,189)
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Real estate taxes	0	0	0	0
Acquisition expenses		0	0	0
Depreciation and amortization	0	0	0	0
Total operating expenses	(23,301)	(22,983)	(57,354)	(55,189)
Total operating income	0	0	0	0
Other expenses:
Interest expense	0	0	0	0
Loss on early extinguishment of debt	0	0	0	0
Loss from derivative financial instruments	0	0	0	0
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Segment assets	0	0	0	0
Expenditures for segment assets	0	0	0	0
Real Estate | Operating Segments
Revenues:
Rental revenue	122,391	115,634	360,348	343,155
Intercompany rental revenue	23,301	22,983	57,354	55,189
Tenant expense reimbursement	20,346	19,176	53,889	56,350
Observatory revenue	0	0	0	0
Third-party management and other fees	345	404	1,088	1,372
Other revenue and fees	4,932	2,536	19,861	6,738
Total revenues	171,315	160,733	492,540	462,804
Operating expenses:
Property operating expenses	41,270	38,585	122,009	115,075
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,331	6,994	6,994
General and administrative expenses	12,899	11,798	36,566	35,623
Observatory expenses	0	0	0	0
Real estate taxes	26,901	24,691	76,001	71,773
Acquisition expenses		0	0	98
Depreciation and amortization	38,473	37,421	119,764	115,002
Total operating expenses	121,874	114,826	361,334	344,565
Total operating income	49,441	45,907	131,206	118,239
Other expenses:
Interest expense	(16,890)	(17,939)	(52,109)	(52,758)
Loss on early extinguishment of debt	(2,157)	0	(2,157)	(552)
Loss from derivative financial instruments	0	0	(289)	0
Income before income taxes	30,394	27,968	76,651	64,929
Income tax expense	(264)	(332)	(960)	(1,018)
Net income	30,130	27,636	75,691	63,911
Segment assets	3,582,935	3,636,262	3,582,935	3,636,262
Expenditures for segment assets	40,320	52,828	142,854	140,052
Observatory | Operating Segments
Revenues:
Rental revenue	0	0	0	0
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	0	0	0	0
Observatory revenue	39,306	38,093	94,212	91,112
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	5	0	10
Total revenues	39,306	38,098	94,212	91,122
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	23,301	22,983	57,354	55,189
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	8,648	7,250	23,079	21,900
Real estate taxes	0	0	0	0
Acquisition expenses		0	0	0
Depreciation and amortization	17	186	104	380
Total operating expenses	31,966	30,419	80,537	77,469
Total operating income	7,340	7,679	13,675	13,653
Other expenses:
Interest expense	0	0	0	0
Loss on early extinguishment of debt	0	0	0	0
Loss from derivative financial instruments	0	0	0	0
Income before income taxes	7,340	7,679	13,675	13,653
Income tax expense	(1,981)	(2,418)	(3,373)	(3,322)
Net income	5,359	5,261	10,302	10,331
Segment assets	265,999	246,307	265,999	246,307
Expenditures for segment assets	$ 13,411	$ 0	$ 20,261	$ 47