Segment Reporting - Components of Segment Profit for Each Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Revenues:
Total revenues	$ 178,529	$ 176,349	$ 345,800	$ 340,682
Operating expenses:
Property operating expenses	39,418	38,529	83,603	80,739
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	2,332	4,663	4,663
General and administrative expenses	13,225	12,579	25,853	23,667
Observatory expenses	7,678	7,176	15,014	14,431
Real estate taxes	26,743	24,542	53,487	49,100
Depreciation and amortization	39,468	40,532	79,351	81,378
Total operating expenses	128,864	125,690	261,971	253,978
Total operating income	49,665	50,659	83,829	86,704
Other income (expense):
Interest income	2,499	775	3,724	1,396
Interest expense	(20,525)	(17,477)	(38,116)	(35,219)
Loss from derivative financial instruments	0	(42)	0	(289)
Income before income taxes	31,639	33,915	49,437	52,592
Income tax (expense) benefit	(1,455)	(2,556)	(1,195)	(2,088)
Net income	30,184	31,359	48,242	50,504
Segment assets	4,162,905	3,831,201	4,162,905	3,831,201	$ 3,931,347
Expenditures for segment assets	76,889	61,324	123,239	109,384
Intersegment Elimination
Revenues:
Total revenues	(21,990)	(20,675)	(35,551)	(34,053)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(21,990)	(20,675)	(35,551)	(34,053)
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
Depreciation and amortization	0	0	0	0
Total operating expenses	(21,990)	(20,675)	(35,551)	(34,053)
Total operating income	0	0	0	0
Other income (expense):
Interest income	0	0	0	0
Interest expense	0	0	0	0
Loss from derivative financial instruments		0		0
Income before income taxes	0	0	0	0
Income tax (expense) benefit	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:
Total revenues	165,318	163,058	324,901	319,829
Operating expenses:
Property operating expenses	39,418	38,529	83,603	80,739
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	2,332	4,663	4,663
General and administrative expenses	13,225	12,579	25,853	23,667
Observatory expenses	0	0	0	0
Real estate taxes	26,743	24,542	53,487	49,100
Depreciation and amortization	39,452	40,458	79,317	81,291
Total operating expenses	121,170	118,440	246,923	239,460
Total operating income	44,148	44,618	77,978	80,369
Other income (expense):
Interest income	2,499	775	3,724	1,396
Interest expense	(20,525)	(17,477)	(38,116)	(35,219)
Loss from derivative financial instruments		(42)		(289)
Income before income taxes	26,122	27,874	43,586	46,257
Income tax (expense) benefit	(268)	(413)	(575)	(696)
Net income	25,854	27,461	43,011	45,561
Segment assets	3,902,676	3,579,873	3,902,676	3,579,873
Expenditures for segment assets	61,017	54,474	95,816	102,534
Observatory | Operating Segments
Revenues:
Total revenues	35,201	33,966	56,450	54,906
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	21,990	20,675	35,551	34,053
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	7,678	7,176	15,014	14,431
Real estate taxes	0	0	0	0
Depreciation and amortization	16	74	34	87
Total operating expenses	29,684	27,925	50,599	48,571
Total operating income	5,517	6,041	5,851	6,335
Other income (expense):
Interest income	0	0	0	0
Interest expense	0	0	0	0
Loss from derivative financial instruments		0		0
Income before income taxes	5,517	6,041	5,851	6,335
Income tax (expense) benefit	(1,187)	(2,143)	(620)	(1,392)
Net income	4,330	3,898	5,231	4,943
Segment assets	260,229	251,328	260,229	251,328
Expenditures for segment assets	15,872	6,850	27,423	6,850
Rental Revenue
Revenues:
Total revenues	124,038	120,844	246,349	237,957
Rental Revenue | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Rental Revenue | Real Estate | Operating Segments
Revenues:
Total revenues	124,038	120,844	246,349	237,957
Rental Revenue | Observatory | Operating Segments
Revenues:
Total revenues	0	0	0	0
Intercompany Rental Revenue
Revenues:
Total revenues	0	0	0	0
Intercompany Rental Revenue | Intersegment Elimination
Revenues:
Total revenues	(21,990)	(20,675)	(35,551)	(34,053)
Intercompany Rental Revenue | Real Estate | Operating Segments
Revenues:
Total revenues	21,990	20,675	35,551	34,053
Intercompany Rental Revenue | Observatory | Operating Segments
Revenues:
Total revenues	0	0	0	0
Tenant Expense Reimbursement
Revenues:
Total revenues	16,205	17,569	33,999	33,543
Tenant Expense Reimbursement | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Tenant Expense Reimbursement | Real Estate | Operating Segments
Revenues:
Total revenues	16,205	17,569	33,999	33,543
Tenant Expense Reimbursement | Observatory | Operating Segments
Revenues:
Total revenues	0	0	0	0
Observatory Revenue
Revenues:
Total revenues	35,201	33,966	56,450	54,906
Observatory Revenue | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Observatory Revenue | Real Estate | Operating Segments
Revenues:
Total revenues	0	0	0	0
Observatory Revenue | Observatory | Operating Segments
Revenues:
Total revenues	35,201	33,966	56,450	54,906
Lease Termination Fees
Revenues:
Total revenues	357	1,251	979	9,189
Lease Termination Fees | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Lease Termination Fees | Real Estate | Operating Segments
Revenues:
Total revenues	357	1,251	979	9,189
Lease Termination Fees | Observatory | Operating Segments
Revenues:
Total revenues	0	0	0	0
Third-Party Management And Other Fees
Revenues:
Total revenues	376	392	839	743
Third-Party Management And Other Fees | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Third-Party Management And Other Fees | Real Estate | Operating Segments
Revenues:
Total revenues	376	392	839	743
Third-Party Management And Other Fees | Observatory | Operating Segments
Revenues:
Total revenues	0	0	0	0
Other Revenue And Fees
Revenues:
Total revenues	2,352	2,327	7,184	4,344
Other Revenue And Fees | Intersegment Elimination
Revenues:
Total revenues	0	0	0	0
Other Revenue And Fees | Real Estate | Operating Segments
Revenues:
Total revenues	2,352	2,327	7,184	4,344
Other Revenue And Fees | Observatory | Operating Segments
Revenues:
Total revenues	$ 0	$ 0	$ 0	$ 0