Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 1,660,659
Initial Cost
Land	1,776,213
Building & Improvements	5,370,786
Cost Capitalized Subsequent To Acquisition
Land	(1,314)
Building & Improvements	1,368,250
Gross Amount at Which Carried at Close of Period
Land	1,774,899
Building & Improvements	6,739,036
Total	8,513,935	$ 10,206,122	$ 12,743,332	$ 16,681,602
Accumulated Depreciation	2,099,137	$ 2,300,116	$ 2,264,694	$ 2,060,706
420 Lexington Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	300,000
Initial Cost
Land	0
Building & Improvements	107,832
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	225,667
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	333,499
Total	333,499
Accumulated Depreciation	133,978
711 Third Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	19,844
Building & Improvements	42,499
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	73,270
Gross Amount at Which Carried at Close of Period
Land	19,844
Building & Improvements	115,769
Total	135,613
Accumulated Depreciation	45,066
555 W. 57th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	18,846
Building & Improvements	78,704
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	62,242
Gross Amount at Which Carried at Close of Period
Land	18,846
Building & Improvements	140,946
Total	159,792
Accumulated Depreciation	69,817
220 East 42nd Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	50,373
Building & Improvements	203,727
Cost Capitalized Subsequent To Acquisition
Land	635
Building & Improvements	161,705
Gross Amount at Which Carried at Close of Period
Land	51,008
Building & Improvements	365,432
Total	416,440
Accumulated Depreciation	108,450
461 Fifth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	0
Building & Improvements	62,695
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	25,581
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	88,276
Total	88,276
Accumulated Depreciation	29,680
750 Third Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	51,093
Building & Improvements	205,972
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	45,551
Gross Amount at Which Carried at Close of Period
Land	51,093
Building & Improvements	251,523
Total	302,616
Accumulated Depreciation	101,854
625 Madison Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	0
Building & Improvements	246,673
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	44,646
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	291,319
Total	291,319
Accumulated Depreciation	118,380
485 Lexington Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	450,000
Initial Cost
Land	77,517
Building & Improvements	326,825
Cost Capitalized Subsequent To Acquisition
Land	765
Building & Improvements	125,806
Gross Amount at Which Carried at Close of Period
Land	78,282
Building & Improvements	452,631
Total	530,913
Accumulated Depreciation	183,003
609 Fifth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	36,677
Building & Improvements	145,954
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	49,527
Gross Amount at Which Carried at Close of Period
Land	36,677
Building & Improvements	195,481
Total	232,158
Accumulated Depreciation	43,777
810 Seventh Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	114,077
Building & Improvements	476,386
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	74,433
Gross Amount at Which Carried at Close of Period
Land	114,077
Building & Improvements	550,819
Total	664,896
Accumulated Depreciation	176,354
1185 Avenue of the Americas
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	0
Building & Improvements	728,213
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	62,893
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	791,106
Total	791,106
Accumulated Depreciation	265,896
1350 Avenue of the Americas
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	91,038
Building & Improvements	380,744
Cost Capitalized Subsequent To Acquisition
Land	(97)
Building & Improvements	50,773
Gross Amount at Which Carried at Close of Period
Land	90,941
Building & Improvements	431,517
Total	522,458
Accumulated Depreciation	136,853
100 Summit Lake Drive
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	10,526
Building & Improvements	43,109
Cost Capitalized Subsequent To Acquisition
Land	(3,337)
Building & Improvements	(94)
Gross Amount at Which Carried at Close of Period
Land	7,189
Building & Improvements	43,015
Total	50,204
Accumulated Depreciation	18,936
200 Summit Lake Drive
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	11,183
Building & Improvements	47,906
Cost Capitalized Subsequent To Acquisition
Land	(5,321)
Building & Improvements	(9,102)
Gross Amount at Which Carried at Close of Period
Land	5,862
Building & Improvements	38,804
Total	44,666
Accumulated Depreciation	21,203
500 Summit Lake Drive
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	9,777
Building & Improvements	39,048
Cost Capitalized Subsequent To Acquisition
Land	(3,601)
Building & Improvements	(7,875)
Gross Amount at Which Carried at Close of Period
Land	6,176
Building & Improvements	31,173
Total	37,349
Accumulated Depreciation	14,523
360 Hamilton Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	29,497
Building & Improvements	118,250
Cost Capitalized Subsequent To Acquisition
Land	(2,625)
Building & Improvements	8,005
Gross Amount at Which Carried at Close of Period
Land	26,872
Building & Improvements	126,255
Total	153,127
Accumulated Depreciation	43,901
1-6 Landmark Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	100,000
Initial Cost
Land	50,947
Building & Improvements	195,167
Cost Capitalized Subsequent To Acquisition
Land	(23,095)
Building & Improvements	(33,824)
Gross Amount at Which Carried at Close of Period
Land	27,852
Building & Improvements	161,343
Total	189,195
Accumulated Depreciation	79,012
7 Landmark Square
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	2,088
Building & Improvements	7,748
Cost Capitalized Subsequent To Acquisition
Land	(367)
Building & Improvements	669
Gross Amount at Which Carried at Close of Period
Land	1,721
Building & Improvements	8,417
Total	10,138
Accumulated Depreciation	1,539
1010 Washington Boulevard
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	7,747
Building & Improvements	30,423
Cost Capitalized Subsequent To Acquisition
Land	(1,259)
Building & Improvements	2,928
Gross Amount at Which Carried at Close of Period
Land	6,488
Building & Improvements	33,351
Total	39,839
Accumulated Depreciation	12,489
1055 Washington Boulevard
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	13,516
Building & Improvements	53,228
Cost Capitalized Subsequent To Acquisition
Land	(5,130)
Building & Improvements	(9,986)
Gross Amount at Which Carried at Close of Period
Land	8,386
Building & Improvements	43,242
Total	51,628
Accumulated Depreciation	20,382
One Madison Ave
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	172,641
Building & Improvements	654,394
Cost Capitalized Subsequent To Acquisition
Land	905
Building & Improvements	18,411
Gross Amount at Which Carried at Close of Period
Land	173,546
Building & Improvements	672,805
Total	846,351
Accumulated Depreciation	193,033
100 Church Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	213,208
Initial Cost
Land	32,494
Building & Improvements	79,996
Cost Capitalized Subsequent To Acquisition
Land	2,500
Building & Improvements	103,936
Gross Amount at Which Carried at Close of Period
Land	34,994
Building & Improvements	183,932
Total	218,926
Accumulated Depreciation	53,269
125 Park Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	120,900
Building & Improvements	189,714
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	80,884
Gross Amount at Which Carried at Close of Period
Land	120,900
Building & Improvements	270,598
Total	391,498
Accumulated Depreciation	77,542
Williamsburg
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	3,677
Building & Improvements	14,708
Cost Capitalized Subsequent To Acquisition
Land	2,523
Building & Improvements	(4,550)
Gross Amount at Which Carried at Close of Period
Land	6,200
Building & Improvements	10,158
Total	16,358
Accumulated Depreciation	2,127
110 East 42nd Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	34,000
Building & Improvements	46,411
Cost Capitalized Subsequent To Acquisition
Land	2,196
Building & Improvements	31,942
Gross Amount at Which Carried at Close of Period
Land	36,196
Building & Improvements	78,353
Total	114,549
Accumulated Depreciation	17,400
400 East 58th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	39,931
Initial Cost
Land	17,549
Building & Improvements	30,916
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	7,833
Gross Amount at Which Carried at Close of Period
Land	17,549
Building & Improvements	38,749
Total	56,298
Accumulated Depreciation	$ 6,119
Interest in property (as a percent)	90.00%
752 Madison Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost
Land	282,415
Building & Improvements	7,131
Cost Capitalized Subsequent To Acquisition
Land	1,871
Building & Improvements	1,183
Gross Amount at Which Carried at Close of Period
Land	284,286
Building & Improvements	8,314
Total	292,600
Accumulated Depreciation	1,380
762 Madison Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	771
Initial Cost
Land	6,153
Building & Improvements	10,461
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	109
Gross Amount at Which Carried at Close of Period
Land	6,153
Building & Improvements	10,570
Total	16,723
Accumulated Depreciation	$ 1,884
Interest in property (as a percent)	90.00%
19-21 East 65th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost
Land	0
Building & Improvements	7,389
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	1,100
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	8,489
Total	8,489
Accumulated Depreciation	$ 1,228
Interest in property (as a percent)	90.00%
304 Park Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 0
Initial Cost
Land	54,189
Building & Improvements	75,619
Cost Capitalized Subsequent To Acquisition
Land	300
Building & Improvements	15,024
Gross Amount at Which Carried at Close of Period
Land	54,489
Building & Improvements	90,643
Total	145,132
Accumulated Depreciation	19,315
635 Sixth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	24,180
Building & Improvements	37,158
Cost Capitalized Subsequent To Acquisition
Land	163
Building & Improvements	51,103
Gross Amount at Which Carried at Close of Period
Land	24,343
Building & Improvements	88,261
Total	112,604
Accumulated Depreciation	10,931
641 Sixth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	45,668
Building & Improvements	67,316
Cost Capitalized Subsequent To Acquisition
Land	308
Building & Improvements	9,760
Gross Amount at Which Carried at Close of Period
Land	45,976
Building & Improvements	77,076
Total	123,052
Accumulated Depreciation	15,891
1080 Amsterdam
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	35,807
Initial Cost
Land	0
Building & Improvements	27,445
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	20,503
Gross Amount at Which Carried at Close of Period
Land	0
Building & Improvements	47,948
Total	47,948
Accumulated Depreciation	$ 5,441
Interest in property (as a percent)	92.50%
315 West 33rd Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 250,000
Initial Cost
Land	195,834
Building & Improvements	164,429
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	15,133
Gross Amount at Which Carried at Close of Period
Land	195,834
Building & Improvements	179,562
Total	375,396
Accumulated Depreciation	25,397
562 Fifth Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	57,052
Building & Improvements	10,487
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	1,213
Gross Amount at Which Carried at Close of Period
Land	57,052
Building & Improvements	11,700
Total	68,752
Accumulated Depreciation	4,458
719 Seventh Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	50,000
Initial Cost
Land	41,850
Building & Improvements	0
Cost Capitalized Subsequent To Acquisition
Land	(670)
Building & Improvements	46,232
Gross Amount at Which Carried at Close of Period
Land	41,180
Building & Improvements	46,232
Total	87,412
Accumulated Depreciation	$ 3,025
Interest in property (as a percent)	75.00%
115 Spring Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 65,550
Initial Cost
Land	11,078
Building & Improvements	44,799
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	1,850
Gross Amount at Which Carried at Close of Period
Land	11,078
Building & Improvements	46,649
Total	57,727
Accumulated Depreciation	5,248
1640 Flatbush Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	6,226
Building & Improvements	501
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	503
Gross Amount at Which Carried at Close of Period
Land	6,226
Building & Improvements	1,004
Total	7,230
Accumulated Depreciation	50
110 Greene Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	45,120
Building & Improvements	215,470
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	12,923
Gross Amount at Which Carried at Close of Period
Land	45,120
Building & Improvements	228,393
Total	273,513
Accumulated Depreciation	$ 23,683
Interest in property (as a percent)	90.00%
5- 7 Dey Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 111,869
Initial Cost
Land	13,400
Building & Improvements	34,175
Cost Capitalized Subsequent To Acquisition
Land	32,022
Building & Improvements	(6,310)
Gross Amount at Which Carried at Close of Period
Land	45,422
Building & Improvements	27,865
Total	73,287
Accumulated Depreciation	419
30 East 40th Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	4,650
Building & Improvements	20,000
Cost Capitalized Subsequent To Acquisition
Land	2
Building & Improvements	6,654
Gross Amount at Which Carried at Close of Period
Land	4,652
Building & Improvements	26,654
Total	31,306
Accumulated Depreciation	$ 2,017
Interest in property (as a percent)	60.00%
113 Greene Street
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 15,523
Initial Cost
Land	3,446
Building & Improvements	27,542
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	0
Gross Amount at Which Carried at Close of Period
Land	3,446
Building & Improvements	27,542
Total	30,988
Accumulated Depreciation	119
712 Madison Avenue
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	28,000
Initial Cost
Land	7,207
Building & Improvements	47,397
Cost Capitalized Subsequent To Acquisition
Land	0
Building & Improvements	0
Gross Amount at Which Carried at Close of Period
Land	7,207
Building & Improvements	47,397
Total	54,604
Accumulated Depreciation	0
Other
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost
Land	1,738
Building & Improvements	16,225
Cost Capitalized Subsequent To Acquisition
Land	(2)
Building & Improvements	(1)
Gross Amount at Which Carried at Close of Period
Land	1,736
Building & Improvements	16,224
Total	17,960
Accumulated Depreciation	$ 4,068