Schedule III - Real Estate and Accumulated Depreciation - Schedule of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 775,929
Initial Cost to the Company
Land and Development Costs	199,287
Building & Improvements	918,018
Cost Capitalized Subsequent to Acquisition
Improvements	2,016,661
Gross Amount at which Carried
Land and Development Costs	209,162
Buildings & Improvements	2,924,804
Total	3,133,966	$ 3,109,433	$ 2,884,486	$ 2,667,655
Accumulated Depreciation	941,612	$ 862,534	$ 747,304	$ 656,900
111 West 33rd Street, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	13,630
Building & Improvements	244,461
Cost Capitalized Subsequent to Acquisition
Improvements	125,514
Gross Amount at which Carried
Land and Development Costs	13,630
Buildings & Improvements	369,975
Total	383,605
Accumulated Depreciation	70,495
1400 Broadway | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	0
Building & Improvements	96,338
Cost Capitalized Subsequent to Acquisition
Improvements	86,939
Gross Amount at which Carried
Land and Development Costs	0
Buildings & Improvements	183,277
Total	183,277
Accumulated Depreciation	46,694
1333 Broadway, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	158,676
Initial Cost to the Company
Land and Development Costs	91,435
Building & Improvements	120,190
Cost Capitalized Subsequent to Acquisition
Improvements	10,469
Gross Amount at which Carried
Land and Development Costs	91,435
Buildings & Improvements	130,659
Total	222,094
Accumulated Depreciation	29,285
1350 Broadway, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	0
Building & Improvements	102,518
Cost Capitalized Subsequent to Acquisition
Improvements	38,180
Gross Amount at which Carried
Land and Development Costs	0
Buildings & Improvements	140,698
Total	140,698
Accumulated Depreciation	38,967
250 West 57th Street, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	173,835
Initial Cost to the Company
Land and Development Costs	2,117
Building & Improvements	5,041
Cost Capitalized Subsequent to Acquisition
Improvements	163,843
Gross Amount at which Carried
Land and Development Costs	2,117
Buildings & Improvements	168,884
Total	171,001
Accumulated Depreciation	49,958
501 Seventh Avenue, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	1,100
Building & Improvements	2,600
Cost Capitalized Subsequent to Acquisition
Improvements	96,842
Gross Amount at which Carried
Land and Development Costs	1,100
Buildings & Improvements	99,442
Total	100,542
Accumulated Depreciation	45,220
1359 Broadway, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	1,233
Building & Improvements	1,809
Cost Capitalized Subsequent to Acquisition
Improvements	63,075
Gross Amount at which Carried
Land and Development Costs	1,233
Buildings & Improvements	64,884
Total	66,117
Accumulated Depreciation	32,420
350 Fifth Avenue (Empire State Building), New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	21,551
Building & Improvements	38,934
Cost Capitalized Subsequent to Acquisition
Improvements	970,966
Gross Amount at which Carried
Land and Development Costs	21,551
Buildings & Improvements	1,009,900
Total	1,031,451
Accumulated Depreciation	275,648
One Grand Central Place, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	7,240
Building & Improvements	17,490
Cost Capitalized Subsequent to Acquisition
Improvements	268,333
Gross Amount at which Carried
Land and Development Costs	7,222
Buildings & Improvements	285,841
Total	293,063
Accumulated Depreciation	123,509
First Stamford Place, Stamford, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	178,943
Initial Cost to the Company
Land and Development Costs	22,952
Building & Improvements	122,739
Cost Capitalized Subsequent to Acquisition
Improvements	75,458
Gross Amount at which Carried
Land and Development Costs	24,861
Buildings & Improvements	196,288
Total	221,149
Accumulated Depreciation	91,678
One Station Place, Stamford, CT (Metro Center) | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	87,236
Initial Cost to the Company
Land and Development Costs	5,313
Building & Improvements	28,602
Cost Capitalized Subsequent to Acquisition
Improvements	19,581
Gross Amount at which Carried
Land and Development Costs	5,313
Buildings & Improvements	48,183
Total	53,496
Accumulated Depreciation	31,903
383 Main Avenue, Norwalk, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,668
Initial Cost to the Company
Land and Development Costs	2,262
Building & Improvements	12,820
Cost Capitalized Subsequent to Acquisition
Improvements	30,878
Gross Amount at which Carried
Land and Development Costs	2,262
Buildings & Improvements	43,698
Total	45,960
Accumulated Depreciation	15,901
500 Mamaroneck Avenue, Harrison, NY | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	4,571
Building & Improvements	25,915
Cost Capitalized Subsequent to Acquisition
Improvements	26,708
Gross Amount at which Carried
Land and Development Costs	4,571
Buildings & Improvements	52,623
Total	57,194
Accumulated Depreciation	26,760
10 Bank Street, White Plains, NY | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	31,624
Initial Cost to the Company
Land and Development Costs	5,612
Building & Improvements	31,803
Cost Capitalized Subsequent to Acquisition
Improvements	20,833
Gross Amount at which Carried
Land and Development Costs	5,612
Buildings & Improvements	52,636
Total	58,248
Accumulated Depreciation	25,537
10 Union Square, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	49,365
Initial Cost to the Company
Land and Development Costs	5,003
Building & Improvements	12,866
Cost Capitalized Subsequent to Acquisition
Improvements	2,579
Gross Amount at which Carried
Land and Development Costs	5,003
Buildings & Improvements	15,445
Total	20,448
Accumulated Depreciation	8,687
1542 Third Avenue, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,592
Initial Cost to the Company
Land and Development Costs	2,239
Building & Improvements	15,266
Cost Capitalized Subsequent to Acquisition
Improvements	464
Gross Amount at which Carried
Land and Development Costs	2,239
Buildings & Improvements	15,730
Total	17,969
Accumulated Depreciation	8,644
1010 Third Avenue, New York, NY and 77 West 55th Street, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	36,990
Initial Cost to the Company
Land and Development Costs	4,462
Building & Improvements	15,817
Cost Capitalized Subsequent to Acquisition
Improvements	1,251
Gross Amount at which Carried
Land and Development Costs	4,463
Buildings & Improvements	17,067
Total	21,530
Accumulated Depreciation	9,500
69-97 Main Street, Westport, CT | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	2,782
Building & Improvements	15,766
Cost Capitalized Subsequent to Acquisition
Improvements	6,317
Gross Amount at which Carried
Land and Development Costs	2,782
Buildings & Improvements	22,083
Total	24,865
Accumulated Depreciation	8,052
103-107 Main Street, Westport, CT | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	1,243
Building & Improvements	7,043
Cost Capitalized Subsequent to Acquisition
Improvements	360
Gross Amount at which Carried
Land and Development Costs	1,260
Buildings & Improvements	7,386
Total	8,646
Accumulated Depreciation	2,754
Property for development at the Transportation Hub in Stamford, CT | land
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	0
Initial Cost to the Company
Land and Development Costs	4,542
Building & Improvements	0
Cost Capitalized Subsequent to Acquisition
Improvements	8,071
Gross Amount at which Carried
Land and Development Costs	12,508
Buildings & Improvements	105
Total	12,613
Accumulated Depreciation	$ 0