Schedule III - Real Estate and Accumulated Depreciation - Schedule of 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	$ 608,567
Initial Cost to the Company
Land	199,287
Building & Improvements	918,018
Cost Capitalized Subsequent to Acquisition
Improvements	1,767,181
Gross Amount at which Carried
Land	209,183
Buildings & Improvements	2,675,303
Total	2,884,486	$ 2,667,655	$ 2,458,629	$ 2,276,330
Accumulated Depreciation	(747,304)	$ (656,900)	$ (556,546)	$ (465,584)
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	13,630
Building & Improvements	244,461
Cost Capitalized Subsequent to Acquisition
Improvements	106,780
Gross Amount at which Carried
Land	13,630
Buildings & Improvements	351,241
Total	364,871
Accumulated Depreciation	(41,629)
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	0
Building & Improvements	96,338
Cost Capitalized Subsequent to Acquisition
Improvements	40,261
Gross Amount at which Carried
Land	0
Buildings & Improvements	136,599
Total	136,599
Accumulated Depreciation	(31,601)
1333 Broadway, New York, NY | office/ retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	158,484
Initial Cost to the Company
Land	91,435
Building & Improvements	120,190
Cost Capitalized Subsequent to Acquisition
Improvements	7,491
Gross Amount at which Carried
Land	91,435
Buildings & Improvements	127,681
Total	219,116
Accumulated Depreciation	(22,331)
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	0
Building & Improvements	102,518
Cost Capitalized Subsequent to Acquisition
Improvements	27,161
Gross Amount at which Carried
Land	0
Buildings & Improvements	129,679
Total	129,679
Accumulated Depreciation	(25,312)
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	2,117
Building & Improvements	5,041
Cost Capitalized Subsequent to Acquisition
Improvements	141,581
Gross Amount at which Carried
Land	2,117
Buildings & Improvements	146,622
Total	148,739
Accumulated Depreciation	(36,058)
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	1,100
Building & Improvements	2,600
Cost Capitalized Subsequent to Acquisition
Improvements	94,778
Gross Amount at which Carried
Land	1,100
Buildings & Improvements	97,378
Total	98,478
Accumulated Depreciation	(43,164)
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	1,233
Building & Improvements	1,809
Cost Capitalized Subsequent to Acquisition
Improvements	57,938
Gross Amount at which Carried
Land	1,233
Buildings & Improvements	59,747
Total	60,980
Accumulated Depreciation	(26,549)
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	21,551
Building & Improvements	38,934
Cost Capitalized Subsequent to Acquisition
Improvements	895,989
Gross Amount at which Carried
Land	21,551
Buildings & Improvements	934,923
Total	956,474
Accumulated Depreciation	(211,068)
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	7,240
Building & Improvements	17,490
Cost Capitalized Subsequent to Acquisition
Improvements	241,218
Gross Amount at which Carried
Land	7,222
Buildings & Improvements	258,726
Total	265,948
Accumulated Depreciation	(109,502)
First Stamford Place, Stamford, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	178,616
Initial Cost to the Company
Land	22,952
Building & Improvements	122,739
Cost Capitalized Subsequent to Acquisition
Improvements	63,292
Gross Amount at which Carried
Land	24,862
Buildings & Improvements	184,121
Total	208,983
Accumulated Depreciation	(78,570)
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	91,592
Initial Cost to the Company
Land	5,313
Building & Improvements	28,602
Cost Capitalized Subsequent to Acquisition
Improvements	15,301
Gross Amount at which Carried
Land	5,313
Buildings & Improvements	43,903
Total	49,216
Accumulated Depreciation	(30,763)
383 Main Avenue, Norwalk, CT | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,614
Initial Cost to the Company
Land	2,262
Building & Improvements	12,820
Cost Capitalized Subsequent to Acquisition
Improvements	22,253
Gross Amount at which Carried
Land	2,262
Buildings & Improvements	35,073
Total	37,335
Accumulated Depreciation	(12,876)
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	4,571
Building & Improvements	25,915
Cost Capitalized Subsequent to Acquisition
Improvements	22,198
Gross Amount at which Carried
Land	4,571
Buildings & Improvements	48,113
Total	52,684
Accumulated Depreciation	(23,147)
10 Bank Street, White Plains, NY | office
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	33,316
Initial Cost to the Company
Land	5,612
Building & Improvements	31,803
Cost Capitalized Subsequent to Acquisition
Improvements	18,412
Gross Amount at which Carried
Land	5,612
Buildings & Improvements	50,215
Total	55,827
Accumulated Depreciation	(21,134)
10 Union Square, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	49,116
Initial Cost to the Company
Land	5,003
Building & Improvements	12,866
Cost Capitalized Subsequent to Acquisition
Improvements	1,966
Gross Amount at which Carried
Land	5,003
Buildings & Improvements	14,832
Total	19,835
Accumulated Depreciation	(7,801)
1542 Third Avenue, New York, NY | retail
SEC Schedule, 12-28, Real Estate Companies, Investment in Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	29,459
Initial Cost to the Company
Land	2,239
Building & Improvements	15,266
Cost Capitalized Subsequent to Acquisition
Improvements	425
Gross Amount at which Carried
Land	2,239
Buildings & Improvements	15,691
Total	17,930
Accumulated Depreciation	(7,774)
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	38,370
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	(8,590)
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	2,782
Building & Improvements	15,766
Cost Capitalized Subsequent to Acquisition
Improvements	1,046
Gross Amount at which Carried
Land	2,782
Buildings & Improvements	16,812
Total	19,594
Accumulated Depreciation	(7,113)
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	1,243
Building & Improvements	7,043
Cost Capitalized Subsequent to Acquisition
Improvements	321
Gross Amount at which Carried
Land	1,260
Buildings & Improvements	7,347
Total	8,607
Accumulated Depreciation	(2,322)
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	4,542
Building & Improvements	0
Cost Capitalized Subsequent to Acquisition
Improvements	7,987
Gross Amount at which Carried
Land	12,529
Buildings & Improvements	0
Total	12,529
Accumulated Depreciation	$ 0