Schedule III - Real Estate and Accumulated Depreciation - Schedule of Real Estate and Accumulated Depreciation (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 759,016
Initial Cost to the Company
Land	199,287
Building & Improvements	918,018
Cost Capitalized Subsequent to Acquisition
Improvements	1,341,324
Gross Amount at which Carried
Land	209,147
Buildings & Improvements	2,249,482
Total	2,458,629	$ 2,276,330	$ 2,139,863	$ 1,649,423
Accumulated Depreciation	(556,546)	$ (465,584)	$ (377,552)	$ (295,351)
111 West 33rd Street, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	87,656
Initial Cost to the Company
Land	13,630
Building & Improvements	244,461
Cost Capitalized Subsequent to Acquisition
Improvements	46,954
Gross Amount at which Carried
Land	13,630
Buildings & Improvements	291,415
Total	305,045
Accumulated Depreciation	(20,781)
1400 Broadway [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	79,237
Initial Cost to the Company
Land	0
Building & Improvements	96,338
Cost Capitalized Subsequent to Acquisition
Improvements	19,908
Gross Amount at which Carried
Land	0
Buildings & Improvements	116,246
Total	116,246
Accumulated Depreciation	(19,003)
1333 Broadway, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	69,520
Initial Cost to the Company
Land	91,435
Building & Improvements	120,190
Cost Capitalized Subsequent to Acquisition
Improvements	4,965
Gross Amount at which Carried
Land	91,435
Buildings & Improvements	125,155
Total	216,590
Accumulated Depreciation	(14,064)
1350 Broadway, New York, NY [Member] | Office/Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	38,842
Initial Cost to the Company
Land	0
Building & Improvements	102,518
Cost Capitalized Subsequent to Acquisition
Improvements	18,986
Gross Amount at which Carried
Land	0
Buildings & Improvements	121,504
Total	121,504
Accumulated Depreciation	(14,154)
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	95,233
Gross Amount at which Carried
Land	2,117
Buildings & Improvements	100,274
Total	102,391
Accumulated Depreciation	(27,130)
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	89,650
Gross Amount at which Carried
Land	1,100
Buildings & Improvements	92,250
Total	93,350
Accumulated Depreciation	(36,636)
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	52,461
Gross Amount at which Carried
Land	1,233
Buildings & Improvements	54,270
Total	55,503
Accumulated Depreciation	(21,296)
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	691,784
Gross Amount at which Carried
Land	21,551
Buildings & Improvements	730,718
Total	752,269
Accumulated Depreciation	(137,791)
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	201,322
Gross Amount at which Carried
Land	7,222
Buildings & Improvements	218,830
Total	226,052
Accumulated Depreciation	(87,075)
First Stamford Place, Stamford, CT [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	234,953
Initial Cost to the Company
Land	22,952
Building & Improvements	122,739
Cost Capitalized Subsequent to Acquisition
Improvements	46,409
Gross Amount at which Carried
Land	24,862
Buildings & Improvements	167,238
Total	192,100
Accumulated Depreciation	(70,988)
One Station Place, Stamford, CT (Metro Center) [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	95,574
Initial Cost to the Company
Land	5,313
Building & Improvements	28,602
Cost Capitalized Subsequent to Acquisition
Improvements	13,069
Gross Amount at which Carried
Land	5,313
Buildings & Improvements	41,671
Total	46,984
Accumulated Depreciation	(27,860)
383 Main Avenue, Norwalk, CT [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	28,639
Initial Cost to the Company
Land	2,262
Building & Improvements	12,820
Cost Capitalized Subsequent to Acquisition
Improvements	15,281
Gross Amount at which Carried
Land	2,262
Buildings & Improvements	28,101
Total	30,363
Accumulated Depreciation	(10,776)
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	18,784
Gross Amount at which Carried
Land	4,571
Buildings & Improvements	44,699
Total	49,270
Accumulated Depreciation	(20,129)
10 Bank Street, White Plains, NY [Member] | Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	31,531
Initial Cost to the Company
Land	5,612
Building & Improvements	31,803
Cost Capitalized Subsequent to Acquisition
Improvements	14,665
Gross Amount at which Carried
Land	5,612
Buildings & Improvements	46,468
Total	52,080
Accumulated Depreciation	(19,174)
10 Union Square, New York, NY [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	48,824
Initial Cost to the Company
Land	5,003
Building & Improvements	12,866
Cost Capitalized Subsequent to Acquisition
Improvements	1,661
Gross Amount at which Carried
Land	5,003
Buildings & Improvements	14,527
Total	19,530
Accumulated Depreciation	(6,954)
1542 Third Avenue, New York, NY [Member] | Retail [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	17,784
Initial Cost to the Company
Land	2,239
Building & Improvements	15,266
Cost Capitalized Subsequent to Acquisition
Improvements	391
Gross Amount at which Carried
Land	2,239
Buildings & Improvements	15,657
Total	17,896
Accumulated Depreciation	(6,925)
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	26,456
Initial Cost to the Company
Land	4,462
Building & Improvements	15,817
Cost Capitalized Subsequent to Acquisition
Improvements	774
Gross Amount at which Carried
Land	4,462
Buildings & Improvements	16,591
Total	21,053
Accumulated Depreciation	(7,740)
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	918
Gross Amount at which Carried
Land	2,782
Buildings & Improvements	16,684
Total	19,466
Accumulated Depreciation	(6,165)
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	158
Gross Amount at which Carried
Land	1,260
Buildings & Improvements	7,184
Total	8,444
Accumulated Depreciation	(1,905)
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	7,951
Gross Amount at which Carried
Land	12,493
Buildings & Improvements	0
Total	12,493
Accumulated Depreciation	$ 0