Segment Reporting Reportable Segments (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) segment	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Rental revenue	$ 112,866	$ 92,210	$ 222,924	$ 182,414
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	18,582	14,304	36,782	29,457
Observatory revenue	30,600	30,389	48,823	47,690
Construction revenue	374	12,963	1,981	27,926
Third-party management and other fees	594	753	1,040	1,364
Other revenue and fees	1,757	4,549	5,105	6,623
Total revenues	164,773	155,168	316,655	295,474
Property operating expenses	37,935	35,149	80,387	71,009
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	447	4,663	898
Marketing, general, and administrative expenses	9,113	9,560	18,213	19,715
Observatory expenses	7,420	7,120	14,397	14,101
Construction expenses	353	12,795	3,222	27,078
Real estate taxes	22,952	18,186	45,930	36,559
Acquisition expenses	0	735	0	735
Depreciation and amortization	39,629	28,637	81,047	58,752
Total operating expenses	119,734	112,629	247,859	228,847
Total operating income	45,039	42,539	68,796	66,627
Interest expense	(17,571)	(14,629)	(33,618)	(28,966)
Income before income taxes	27,468	27,910	35,178	37,661
Income tax (expense) benefit	(883)	(2,629)	(705)	(1,149)
Net income	26,585	25,281	34,473	36,512
Segment assets	3,293,964	2,476,625	3,293,964	2,476,625	$ 3,296,495
Expenditures for segment assets	31,820	37,735	$ 69,213	54,978
Number of reportable segments | segment			2
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Rental revenue	0	0	$ 0	0
Intercompany rental revenue	(18,910)	(16,298)	(30,574)	(31,566)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Construction revenue	(1,400)	(901)	(3,715)	(1,863)
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(20,310)	(17,199)	(34,289)	(33,429)
Property operating expenses	0	0	0	0
Intercompany rent expense	(18,910)	(16,298)	(30,574)	(31,566)
Ground rent expenses	0	0	0	0
Marketing, general, and administrative expenses	0	0	0	0
Observatory expenses	0	0	0	0
Construction expenses	(1,213)	(787)	(3,317)	(1,619)
Real estate taxes	0	0	0	0
Acquisition expenses		0		0
Depreciation and amortization	0	0	0	0
Total operating expenses	(20,123)	(17,085)	(33,891)	(33,185)
Total operating income	(187)	(114)	(398)	(244)
Interest expense	0	0	0	0
Income before income taxes	(187)	(114)	(398)	(244)
Income tax (expense) benefit	0	0	0	0
Net income	(187)	(114)	(398)	(244)
Segment assets	0	0	0	0
Expenditures for segment assets	0	0	0	0
Real estate segment | Operating Segments [Member]
Segment Reporting Information [Line Items]
Rental revenue	112,866	92,210	222,924	182,414
Intercompany rental revenue	18,910	16,298	30,574	31,566
Tenant expense reimbursement	18,582	14,304	36,782	29,457
Observatory revenue	0	0	0	0
Construction revenue	0	0	0	0
Third-party management and other fees	594	753	1,040	1,364
Other revenue and fees	1,757	4,549	5,105	6,623
Total revenues	152,709	128,114	296,425	251,424
Property operating expenses	37,935	35,149	80,387	71,009
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,332	447	4,663	898
Marketing, general, and administrative expenses	9,113	9,560	18,213	19,715
Observatory expenses	0	0	0	0
Construction expenses	0	0	0	0
Real estate taxes	22,952	18,186	45,930	36,559
Acquisition expenses		735		735
Depreciation and amortization	39,542	28,556	80,775	58,608
Total operating expenses	111,874	92,633	229,968	187,524
Total operating income	40,835	35,481	66,457	63,900
Interest expense	(17,571)	(14,629)	(33,618)	(28,966)
Income before income taxes	23,264	20,852	32,839	34,934
Income tax (expense) benefit	(422)	(630)	(773)	(930)
Net income	22,842	20,222	32,066	34,004
Segment assets	3,051,768	2,221,530	3,051,768	2,221,530
Expenditures for segment assets	31,796	37,735	69,189	54,978
Observatory segment | Operating 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	30,600	30,389	48,823	47,690
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	30,600	30,389	48,823	47,690
Property operating expenses	0	0	0	0
Intercompany rent expense	18,910	16,298	30,574	31,566
Ground rent expenses	0	0	0	0
Marketing, general, and administrative expenses	0	0	0	0
Observatory expenses	7,420	7,120	14,397	14,101
Construction expenses	0	0	0	0
Real estate taxes	0	0	0	0
Acquisition expenses		0		0
Depreciation and amortization	87	77	171	137
Total operating expenses	26,417	23,495	45,142	45,804
Total operating income	4,183	6,894	3,681	1,886
Interest expense	0	0	0	0
Income before income taxes	4,183	6,894	3,681	1,886
Income tax (expense) benefit	(528)	(2,381)	(272)	(178)
Net income	3,655	4,513	3,409	1,708
Segment assets	240,374	243,189	240,374	243,189
Expenditures for segment assets	24	0	24	0
Other Segments [Member] | Operating 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	1,774	13,864	5,696	29,789
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	1,774	13,864	5,696	29,789
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	1,566	13,582	6,539	28,697
Real estate taxes	0	0	0	0
Acquisition expenses		0		0
Depreciation and amortization	0	4	101	7
Total operating expenses	1,566	13,586	6,640	28,704
Total operating income	208	278	(944)	1,085
Interest expense	0	0	0	0
Income before income taxes	208	278	(944)	1,085
Income tax (expense) benefit	67	382	340	(41)
Net income	275	660	(604)	1,044
Segment assets	1,822	11,906	1,822	11,906
Expenditures for segment assets	$ 0	$ 0	$ 0	$ 0