Segment Reporting - Components of Segment Profit for Each Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues:
Rental revenue	$ 123,621	$ 122,391	$ 369,970	$ 360,348
Intercompany rental revenue	0	0	0	0
Tenant expense reimbursement	18,627	20,346	52,626	53,889
Observatory revenue	40,241	39,306	96,691	94,212
Lease termination fees	1,185	1,165	2,164	10,354
Third-party management and other fees	312	345	1,151	1,088
Other revenue and fees	2,416	2,993	9,600	7,338
Total revenues	186,402	186,546	532,202	527,229
Operating expenses:
Property operating expenses	42,772	41,270	126,375	122,009
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,331	6,994	6,994
General and administrative expenses	13,148	12,899	39,001	36,566
Observatory expenses	8,854	8,648	23,868	23,079
Real estate taxes	28,284	26,901	81,771	76,001
Depreciation and amortization	42,475	38,490	121,826	119,868
Total operating expenses	137,864	130,539	399,835	384,517
Total operating income	48,538	56,007	132,367	142,712
Other income (expense):
Interest income	3,485	774	7,209	2,169
Interest expense	(20,658)	(16,890)	(58,774)	(52,109)
Loss on early extinguishment of debt	0	(2,157)	0	(2,157)
Loss from derivative financial instruments	0	0	0	(289)
Income before income taxes	31,365	37,734	80,802	90,326
Income tax expense	(2,135)	(2,245)	(3,330)	(4,333)
Net income	29,230	35,489	77,472	85,993
Segment assets	4,166,832	3,848,934	4,166,832	3,848,934	$ 3,931,347
Expenditures for segment assets	67,120	53,731	190,359	163,115
Intersegment Elimination
Revenues:
Rental revenue	0	0	0	0
Intercompany rental revenue	(23,805)	(23,301)	(59,356)	(57,354)
Tenant expense reimbursement	0	0	0	0
Observatory revenue	0	0	0	0
Lease termination fees	0	0	0	0
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	(23,805)	(23,301)	(59,356)	(57,354)
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	(23,805)	(23,301)	(59,356)	(57,354)
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	(23,805)	(23,301)	(59,356)	(57,354)
Total operating income	0	0	0	0
Other income (expense):
Interest income	0	0	0	0
Interest expense	0	0	0	0
Loss on early extinguishment of debt		0		0
Loss from derivative financial instruments				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	123,621	122,391	369,970	360,348
Intercompany rental revenue	23,805	23,301	59,356	57,354
Tenant expense reimbursement	18,627	20,346	52,626	53,889
Observatory revenue	0	0	0	0
Lease termination fees	1,185	1,165	2,164	10,354
Third-party management and other fees	312	345	1,151	1,088
Other revenue and fees	2,416	2,993	9,600	7,338
Total revenues	169,966	170,541	494,867	490,371
Operating expenses:
Property operating expenses	42,772	41,270	126,375	122,009
Intercompany rent expense	0	0	0	0
Ground rent expenses	2,331	2,331	6,994	6,994
General and administrative expenses	13,148	12,899	39,001	36,566
Observatory expenses	0	0	0	0
Real estate taxes	28,284	26,901	81,771	76,001
Depreciation and amortization	42,458	38,473	121,775	119,764
Total operating expenses	128,993	121,874	375,916	361,334
Total operating income	40,973	48,667	118,951	129,037
Other income (expense):
Interest income	3,485	774	7,209	2,169
Interest expense	(20,658)	(16,890)	(58,774)	(52,109)
Loss on early extinguishment of debt		(2,157)		(2,157)
Loss from derivative financial instruments				(289)
Income before income taxes	23,800	30,394	67,386	76,651
Income tax expense	(277)	(264)	(852)	(960)
Net income	23,523	30,130	66,534	75,691
Segment assets	3,904,454	3,582,935	3,904,454	3,582,935
Expenditures for segment assets	52,613	40,320	148,429	142,854
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	40,241	39,306	96,691	94,212
Lease termination fees	0	0	0	0
Third-party management and other fees	0	0	0	0
Other revenue and fees	0	0	0	0
Total revenues	40,241	39,306	96,691	94,212
Operating expenses:
Property operating expenses	0	0	0	0
Intercompany rent expense	23,805	23,301	59,356	57,354
Ground rent expenses	0	0	0	0
General and administrative expenses	0	0	0	0
Observatory expenses	8,854	8,648	23,868	23,079
Real estate taxes	0	0	0	0
Depreciation and amortization	17	17	51	104
Total operating expenses	32,676	31,966	83,275	80,537
Total operating income	7,565	7,340	13,416	13,675
Other income (expense):
Interest income	0	0	0	0
Interest expense	0	0	0	0
Loss on early extinguishment of debt		0		0
Loss from derivative financial instruments				0
Income before income taxes	7,565	7,340	13,416	13,675
Income tax expense	(1,858)	(1,981)	(2,478)	(3,373)
Net income	5,707	5,359	10,938	10,302
Segment assets	262,378	265,999	262,378	265,999
Expenditures for segment assets	$ 14,507	$ 13,411	$ 41,930	$ 20,261