Properties Held for Sale and Dispositions (Details) (USD $)	3 Months Ended									12 Months Ended			1 Months Ended														0 Months Ended
	Dec. 31, 2014 sqft	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 sqft	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2014 sqft		Jul. 31, 2014 sqft		May 31, 2014 sqft		Sep. 30, 2013 sqft		Aug. 31, 2013 sqft		Feb. 28, 2013 sqft		Feb. 29, 2012 sqft		Jan. 31, 2015
Properties Sold [Abstract]
Area of property (sqft)	33,891,036									33,891,036
Gain (Loss) on Sale	$ 18,817,000	$ 29,507,000	$ 114,735,000	$ 0	$ 0	$ 13,787,000		$ 0	$ 1,113,000	$ 163,059,000	$ 14,900,000	$ 6,627,000
Revenues
Rental revenue										51,090,000	94,558,000	97,254,000
Escalation and reimbursement revenues										4,646,000	14,856,000	16,232,000
Other income										23,000	554,000	8,295,000
Total revenues										55,759,000	109,968,000	121,781,000
Operating expenses										7,772,000	20,568,000	23,166,000
Real estate taxes										7,156,000	16,521,000	16,190,000
Ground rent										3,001,000	7,975,000	6,362,000
Interest expense, net of interest income										12,652,000	19,782,000	22,298,000
Depreciable real estate reserves										0	2,150,000	0
Transaction related costs, net of recoveries										89,000	2,000	383,000
Depreciation and amortization										5,581,000	16,443,000	20,250,000
Amortization of deferred financing costs										433,000	840,000	892,000
Total expenses										36,684,000	84,281,000	89,541,000
Net income from discontinued operations	3,626,000	4,035,000	5,645,000	5,769,000	5,836,000	7,435,000		5,943,000	6,473,000	19,075,000	25,687,000	32,240,000
180 Maiden Lane
Properties Sold [Abstract]
Area of property (sqft)	1,090,000									1,090,000
2 Herald Square
Properties Sold [Abstract]
Area of property (sqft)													354,400	[1]
Sales Price													365,000,000	[1]
Gain (Loss) on Sale													18,800,000	[1]
Employee compensation award													2,500,000
985-987 Third Avenue
Properties Sold [Abstract]
Area of property (sqft)															13,678	[1]
Sales Price															68,700,000	[1]
Gain (Loss) on Sale															29,800,000	[1]
Employee compensation award															1,300,000
673 First Avenue
Properties Sold [Abstract]
Area of property (sqft)																	422,000	[1]
Sales Price																	145,000,000	[1]
Gain (Loss) on Sale																	117,600,000	[1]
Employee compensation award																	3,400,000
300 Main Street
Properties Sold [Abstract]
Area of property (sqft)						130,000	[1]												130,000	[1]
Sales Price																			13,500,000	[1]
Gain (Loss) on Sale																			(2,200,000)	[1]
333 West 34th Street
Properties Sold [Abstract]
Area of property (sqft)																					345,400	[1]
Sales Price																					220,300,000	[1]
Gain (Loss) on Sale																					13,800,000	[1]
Employee compensation award																					3,000,000
44 West 55th Street
Properties Sold [Abstract]
Area of property (sqft)																							8,557	[1]
Sales Price																							6,300,000	[1]
Gain (Loss) on Sale																							1,100,000	[1]
292 Madison Avenue
Properties Sold [Abstract]
Area of property (sqft)																									203,800	[1]
Sales Price																									85,000,000	[1]
Gain (Loss) on Sale																									6,600,000	[1]
Employee compensation award																									1,500,000
Subsequent Event | 180 Maiden Lane
Properties Sold [Abstract]
Sales Price																											$ 470,000,000

[1] The gain on sale for 2 Herald Square, 985-987 Third Avenue, 673 First Avenue, 333 West 34th Street, and 292 Madison Avenue are net of a $2.5 million, $1.3 million, $3.4 million, $3.0 million and $1.5 million employee compensation award, respectively, accrued in connection with the realization of this investment gain as a bonus to certain employees that were instrumental in realizing the gain on sale.