Schedule III - Real Estate and Accumulated Depreciation - Schedule of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 605,542
Initial Cost to the Company
Land and Development Costs	199,287
Building & Improvements	918,018
Cost Capitalized Subsequent to Acquisition
Improvements	1,992,128
Gross Amount at which Carried
Land and Development Costs	209,185
Buildings & Improvements	2,900,248
Total	3,109,433	$ 2,884,486	$ 2,667,655	$ 2,458,629
Accumulated Depreciation	(862,534)	$ (747,304)	$ (656,900)	$ (556,546)
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	124,153
Gross Amount at which Carried
Land and Development Costs	13,630
Buildings & Improvements	368,614
Total	382,244
Accumulated Depreciation	(54,356)
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	76,764
Gross Amount at which Carried
Land and Development Costs	0
Buildings & Improvements	173,102
Total	173,102
Accumulated Depreciation	(37,745)
1333 Broadway, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	158,617
Initial Cost to the Company
Land and Development Costs	91,435
Building & Improvements	120,190
Cost Capitalized Subsequent to Acquisition
Improvements	9,434
Gross Amount at which Carried
Land and Development Costs	91,435
Buildings & Improvements	129,624
Total	221,059
Accumulated Depreciation	(25,483)
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	32,818
Gross Amount at which Carried
Land and Development Costs	0
Buildings & Improvements	135,336
Total	135,336
Accumulated Depreciation	(32,584)
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	0
Initial Cost to the Company
Land and Development Costs	2,117
Building & Improvements	5,041
Cost Capitalized Subsequent to Acquisition
Improvements	153,241
Gross Amount at which Carried
Land and Development Costs	2,117
Buildings & Improvements	158,282
Total	160,399
Accumulated Depreciation	(42,521)
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	94,547
Gross Amount at which Carried
Land and Development Costs	1,100
Buildings & Improvements	97,147
Total	98,247
Accumulated Depreciation	(40,924)
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	59,570
Gross Amount at which Carried
Land and Development Costs	1,233
Buildings & Improvements	61,379
Total	62,612
Accumulated Depreciation	(29,437)
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	994,622
Gross Amount at which Carried
Land and Development Costs	21,551
Buildings & Improvements	1,033,556
Total	1,055,107
Accumulated Depreciation	(262,236)
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	266,090
Gross Amount at which Carried
Land and Development Costs	7,222
Buildings & Improvements	283,598
Total	290,820
Accumulated Depreciation	(119,574)
First Stamford Place, Stamford, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	178,820
Initial Cost to the Company
Land and Development Costs	22,952
Building & Improvements	122,739
Cost Capitalized Subsequent to Acquisition
Improvements	73,243
Gross Amount at which Carried
Land and Development Costs	24,862
Buildings & Improvements	194,072
Total	218,934
Accumulated Depreciation	(86,636)
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	89,474
Initial Cost to the Company
Land and Development Costs	5,313
Building & Improvements	28,602
Cost Capitalized Subsequent to Acquisition
Improvements	18,876
Gross Amount at which Carried
Land and Development Costs	5,313
Buildings & Improvements	47,478
Total	52,791
Accumulated Depreciation	(32,379)
383 Main Avenue, Norwalk, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,647
Initial Cost to the Company
Land and Development Costs	2,262
Building & Improvements	12,820
Cost Capitalized Subsequent to Acquisition
Improvements	28,083
Gross Amount at which Carried
Land and Development Costs	2,262
Buildings & Improvements	40,903
Total	43,165
Accumulated Depreciation	(14,815)
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	24,723
Gross Amount at which Carried
Land and Development Costs	4,571
Buildings & Improvements	50,638
Total	55,209
Accumulated Depreciation	(24,714)
10 Bank Street, White Plains, NY | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	32,496
Initial Cost to the Company
Land and Development Costs	5,612
Building & Improvements	31,803
Cost Capitalized Subsequent to Acquisition
Improvements	19,699
Gross Amount at which Carried
Land and Development Costs	5,612
Buildings & Improvements	51,502
Total	57,114
Accumulated Depreciation	(23,557)
10 Union Square, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	49,252
Initial Cost to the Company
Land and Development Costs	5,003
Building & Improvements	12,866
Cost Capitalized Subsequent to Acquisition
Improvements	2,363
Gross Amount at which Carried
Land and Development Costs	5,003
Buildings & Improvements	15,229
Total	20,232
Accumulated Depreciation	(8,245)
1542 Third Avenue, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,532
Initial Cost to the Company
Land and Development Costs	2,239
Building & Improvements	15,266
Cost Capitalized Subsequent to Acquisition
Improvements	449
Gross Amount at which Carried
Land and Development Costs	2,239
Buildings & Improvements	15,715
Total	17,954
Accumulated Depreciation	(8,210)
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	37,704
Initial Cost to the Company
Land and Development Costs	4,462
Building & Improvements	15,817
Cost Capitalized Subsequent to Acquisition
Improvements	1,173
Gross Amount at which Carried
Land and Development Costs	4,462
Buildings & Improvements	16,990
Total	21,452
Accumulated Depreciation	(9,037)
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	3,933
Gross Amount at which Carried
Land and Development Costs	2,782
Buildings & Improvements	19,699
Total	22,481
Accumulated Depreciation	(7,542)
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	358
Gross Amount at which Carried
Land and Development Costs	1,260
Buildings & Improvements	7,384
Total	8,644
Accumulated Depreciation	(2,539)
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	7,989
Gross Amount at which Carried
Land and Development Costs	12,531
Buildings & Improvements	0
Total	12,531
Accumulated Depreciation	$ 0