Schedule III - Real Estate and Accumulated Depreciation - Schedule of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 717,164
Initial Cost to the Company
Land	199,287
Building & Improvements	918,018
Cost Capitalized Subsequent to Acquisition
Improvements	1,550,350
Gross Amount at which Carried
Land	209,182
Buildings & Improvements	2,458,473
Total	2,667,655	$ 2,458,629	$ 2,276,330	$ 2,139,863
Accumulated Depreciation	(656,900)	$ (556,546)	$ (465,584)	$ (377,552)
111 West 33rd Street, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	84,085
Initial Cost to the Company
Land	13,630
Building & Improvements	244,461
Cost Capitalized Subsequent to Acquisition
Improvements	76,316
Gross Amount at which Carried
Land	13,630
Buildings & Improvements	320,777
Total	334,407
Accumulated Depreciation	(31,591)
1400 Broadway [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	76,085
Initial Cost to the Company
Land	0
Building & Improvements	96,338
Cost Capitalized Subsequent to Acquisition
Improvements	28,738
Gross Amount at which Carried
Land	0
Buildings & Improvements	125,076
Total	125,076
Accumulated Depreciation	(27,287)
1333 Broadway, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	66,590
Initial Cost to the Company
Land	91,435
Building & Improvements	120,190
Cost Capitalized Subsequent to Acquisition
Improvements	6,143
Gross Amount at which Carried
Land	91,435
Buildings & Improvements	126,333
Total	217,768
Accumulated Depreciation	(18,481)
1350 Broadway, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	37,410
Initial Cost to the Company
Land	0
Building & Improvements	102,518
Cost Capitalized Subsequent to Acquisition
Improvements	21,601
Gross Amount at which Carried
Land	0
Buildings & Improvements	124,119
Total	124,119
Accumulated Depreciation	(19,330)
250 West 57th Street, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	2,117
Building & Improvements	5,041
Cost Capitalized Subsequent to Acquisition
Improvements	113,414
Gross Amount at which Carried
Land	2,117
Buildings & Improvements	118,455
Total	120,572
Accumulated Depreciation	(32,729)
501 Seventh Avenue, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	1,100
Building & Improvements	2,600
Cost Capitalized Subsequent to Acquisition
Improvements	92,032
Gross Amount at which Carried
Land	1,100
Buildings & Improvements	94,632
Total	95,732
Accumulated Depreciation	(39,550)
1359 Broadway, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	1,233
Building & Improvements	1,809
Cost Capitalized Subsequent to Acquisition
Improvements	55,853
Gross Amount at which Carried
Land	1,233
Buildings & Improvements	57,662
Total	58,895
Accumulated Depreciation	(24,867)
350 Fifth Avenue (Empire State Building), New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	21,551
Building & Improvements	38,934
Cost Capitalized Subsequent to Acquisition
Improvements	803,025
Gross Amount at which Carried
Land	21,551
Buildings & Improvements	841,959
Total	863,510
Accumulated Depreciation	(174,484)
One Grand Central Place, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	7,240
Building & Improvements	17,490
Cost Capitalized Subsequent to Acquisition
Improvements	216,947
Gross Amount at which Carried
Land	7,222
Buildings & Improvements	234,455
Total	241,677
Accumulated Depreciation	(98,280)
First Stamford Place, Stamford, CT [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	178,378
Initial Cost to the Company
Land	22,952
Building & Improvements	122,739
Cost Capitalized Subsequent to Acquisition
Improvements	53,718
Gross Amount at which Carried
Land	24,862
Buildings & Improvements	174,547
Total	199,409
Accumulated Depreciation	(76,114)
One Station Place, Stamford, CT (Metro Center) [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	93,606
Initial Cost to the Company
Land	5,313
Building & Improvements	28,602
Cost Capitalized Subsequent to Acquisition
Improvements	13,489
Gross Amount at which Carried
Land	5,313
Buildings & Improvements	42,091
Total	47,404
Accumulated Depreciation	(29,225)
383 Main Avenue, Norwalk, CT [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,572
Initial Cost to the Company
Land	2,262
Building & Improvements	12,820
Cost Capitalized Subsequent to Acquisition
Improvements	17,534
Gross Amount at which Carried
Land	2,262
Buildings & Improvements	30,354
Total	32,616
Accumulated Depreciation	(11,933)
500 Mamaroneck Avenue, Harrison, NY [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	4,571
Building & Improvements	25,915
Cost Capitalized Subsequent to Acquisition
Improvements	22,393
Gross Amount at which Carried
Land	4,571
Buildings & Improvements	48,308
Total	52,879
Accumulated Depreciation	(21,928)
10 Bank Street, White Plains, NY [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	34,092
Initial Cost to the Company
Land	5,612
Building & Improvements	31,803
Cost Capitalized Subsequent to Acquisition
Improvements	17,042
Gross Amount at which Carried
Land	5,612
Buildings & Improvements	48,845
Total	54,457
Accumulated Depreciation	(19,441)
10 Union Square, New York, NY [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	48,966
Initial Cost to the Company
Land	5,003
Building & Improvements	12,866
Cost Capitalized Subsequent to Acquisition
Improvements	1,787
Gross Amount at which Carried
Land	5,003
Buildings & Improvements	14,653
Total	19,656
Accumulated Depreciation	(7,359)
1542 Third Avenue, New York, NY [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,384
Initial Cost to the Company
Land	2,239
Building & Improvements	15,266
Cost Capitalized Subsequent to Acquisition
Improvements	424
Gross Amount at which Carried
Land	2,239
Buildings & Improvements	15,690
Total	17,929
Accumulated Depreciation	(7,347)
1010 Third Avenue, New York, NY and 77 West 55th Street, New York, NY [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	38,996
Initial Cost to the Company
Land	4,462
Building & Improvements	15,817
Cost Capitalized Subsequent to Acquisition
Improvements	783
Gross Amount at which Carried
Land	4,462
Buildings & Improvements	16,600
Total	21,062
Accumulated Depreciation	(8,166)
69-97 Main Street, Westport, CT [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	2,782
Building & Improvements	15,766
Cost Capitalized Subsequent to Acquisition
Improvements	943
Gross Amount at which Carried
Land	2,782
Buildings & Improvements	16,709
Total	19,491
Accumulated Depreciation	(6,679)
103-107 Main Street, Westport, CT [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	1,243
Building & Improvements	7,043
Cost Capitalized Subsequent to Acquisition
Improvements	182
Gross Amount at which Carried
Land	1,260
Buildings & Improvements	7,208
Total	8,468
Accumulated Depreciation	(2,109)
Property for development at the Transportation Hub, Stamford, CT [Member] | Land [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land	4,542
Building & Improvements	0
Cost Capitalized Subsequent to Acquisition
Improvements	7,986
Gross Amount at which Carried
Land	12,528
Buildings & Improvements	0
Total	12,528
Accumulated Depreciation	$ 0