Segment Reporting (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) segment	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Number of reportable segments | segment			2
Segment Reporting Information [Line Items]
Rental revenue	$ 110,903	$ 106,152	$ 333,827	$ 288,566
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	23,096	20,034	59,878	49,491
Observatory revenue	35,702	35,684	84,525	83,374
Construction revenue	0	5,804	1,981	33,730
Third-party management and other fees	618	561	1,658	1,925
Other revenue and fees	5,460	1,206	10,565	7,829
Total revenues	175,779	169,441	492,434	464,915
Operating expenses:
Property operating expenses	41,664	38,291	122,051	109,300
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,066	6,994	2,964
Marketing, general, and administrative expenses	10,182	10,071	28,395	29,786
Observatory expenses	7,284	7,109	21,681	21,210
Construction expenses	0	6,095	3,222	33,173
Real estate taxes	23,613	21,870	69,543	58,429
Acquisition expenses	193	2,647	193	3,382
Depreciation and amortization	45,169	37,880	126,216	96,632
Total operating expenses	130,436	126,029	378,295	354,876
Total operating income	45,343	43,412	114,139	110,039
Interest expense	(16,680)	(17,674)	(50,298)	(46,640)
Income before income taxes	28,663	25,738	63,841	63,399
Income tax expense	(2,578)	(3,004)	(3,283)	(4,153)
Net income	26,085	22,734	60,558	59,246
Segment assets	3,308,767	3,302,016	3,308,767	3,302,016	$ 3,296,495
Expenditures for segment assets	40,102	422,274	109,315	460,009
Operating Segments [Member] | Real estate segment
Segment Reporting Information [Line Items]
Rental revenue	110,903	106,152	333,827	288,566
Intercompany rental revenue	20,828	19,986	51,402	51,552
Tenant expense reimbursement	23,096	20,034	59,878	49,491
Observatory revenue	0	0	0	0
Construction revenue	0	0	0	0
Third-party management and other fees	618	561	1,658	1,925
Other revenue and fees	5,460	1,206	10,565	7,829
Total revenues	160,905	147,939	457,330	399,363
Operating expenses:
Property operating expenses	41,664	38,291	122,051	109,300
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,066	6,994	2,964
Marketing, general, and administrative expenses	10,182	10,071	28,395	29,786
Observatory expenses	0	0	0	0
Construction expenses	0	0	0	0
Real estate taxes	23,613	21,870	69,543	58,429
Acquisition expenses	193	2,647	193	3,382
Depreciation and amortization	45,072	37,797	125,847	96,404
Total operating expenses	123,055	112,742	353,023	300,265
Total operating income	37,850	35,197	104,307	99,098
Interest expense	(16,680)	(17,674)	(50,298)	(46,640)
Income before income taxes	21,170	17,523	54,009	52,458
Income tax expense	(474)	(94)	(1,247)	(1,024)
Net income	20,696	17,429	52,762	51,434
Segment assets	3,070,234	3,038,363	3,070,234	3,038,363
Expenditures for segment assets	39,915	422,175	109,104	459,910
Operating Segments [Member] | Observatory segment
Segment Reporting Information [Line Items]
Rental revenue	0	0	0	0
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	0	0	0	0
Observatory revenue	35,702	35,684	84,525	83,374
Construction 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	35,702	35,684	84,525	83,374
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	20,828	19,986	51,402	51,552
Ground rent expenses	0	0	0	0
Marketing, general, and administrative expenses	0	0	0	0
Observatory expenses	7,284	7,109	21,681	21,210
Construction expenses	0	0	0	0
Real estate taxes	0	0	0	0
Acquisition expenses	0	0	0	0
Depreciation and amortization	97	79	268	217
Total operating expenses	28,209	27,174	73,351	72,979
Total operating income	7,493	8,510	11,174	10,395
Interest expense	0	0	0	0
Income before income taxes	7,493	8,510	11,174	10,395
Income tax expense	(2,104)	(2,857)	(2,376)	(3,035)
Net income	5,389	5,653	8,798	7,360
Segment assets	235,937	255,081	235,937	255,081
Expenditures for segment assets	187	99	211	99
Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Rental revenue	0	0	0	0
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Construction revenue	0	7,292	5,696	37,081
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	0	7,292	5,696	37,081
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	0	0	0	0
Ground rent expenses	0	0	0	0
Marketing, general, and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Construction expenses	0	7,355	6,539	36,052
Real estate taxes	0	0	0	0
Acquisition expenses	0	0	0	0
Depreciation and amortization	0	4	101	11
Total operating expenses	0	7,359	6,640	36,063
Total operating income	0	(67)	(944)	1,018
Interest expense	0	0	0	0
Income before income taxes	0	(67)	(944)	1,018
Income tax expense	0	(53)	340	(94)
Net income	0	(120)	(604)	924
Segment assets	2,596	8,572	2,596	8,572
Expenditures for segment assets	0	0	0	0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Rental revenue	0	0	0	0
Intercompany rental revenue	(20,828)	(19,986)	(51,402)	(51,552)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Construction revenue	0	(1,488)	(3,715)	(3,351)
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(20,828)	(21,474)	(55,117)	(54,903)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(20,828)	(19,986)	(51,402)	(51,552)
Ground rent expenses	0	0	0	0
Marketing, general, and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Construction expenses	0	(1,260)	(3,317)	(2,879)
Real estate taxes	0	0	0	0
Acquisition expenses	0	0	0	0
Depreciation and amortization	0	0	0	0
Total operating expenses	(20,828)	(21,246)	(54,719)	(54,431)
Total operating income	0	(228)	(398)	(472)
Interest expense	0	0	0	0
Income before income taxes	0	(228)	(398)	(472)
Income tax expense	0	0	0	0
Net income	0	(228)	(398)	(472)
Segment assets	0	0	0	0
Expenditures for segment assets	$ 0	$ 0	$ 0	$ 0