Debt Schedule of Long term Debt (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Total	$ 1,005,569		$ 883,112
Mortgage notes payable
Debt Instrument [Line Items]
Fixed rate mortgage debt	940,835		824,088
Floating rate mortgage debt	41,059		47,331
Total mortgage debt	981,894		871,419
Total principal	1,587,494		1,196,419
Unamortized premiums, net of unamortized discount	10,674		11,693
Total	1,598,168		1,208,112
Mortgage notes payable | 501 Seventh Avenue [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	0	[1]
Stated Rate		[1]
Effective Rate		[1],[2]
Mortgage notes payable | 501 Seventh Avenue - Note 1 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	1,037	[1]
Mortgage notes payable | 501 Seventh Avenue - Note 2 - Tranche 1 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	31,459	[1]
Mortgage notes payable | 501 Seventh Avenue - Note 2 - Tranche 2 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	6,889	[1]
Mortgage notes payable | 1359 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	44,146	[1]	0	[1]
Stated Rate	6.04%
Effective Rate	6.56%	[2]
Mortgage notes payable | 1359 Broadway - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	9,579	[1]
Mortgage notes payable | 1359 Broadway - Second Lien - Tranche 1 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	5,561	[1]
Mortgage notes payable | 1359 Broadway - Second Lien - Tranche 2 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	11,311	[1]
Mortgage notes payable | 1359 Broadway - Second Lien - Tranche 3 [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	0	[1]	18,572	[1]
Mortgage notes payable | One Grand Central Place - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	69,994		71,723
Stated Rate	5.34%
Effective Rate	6.85%	[2]
Mortgage notes payable | One Grand Central Place - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	14,642		14,884
Stated Rate	7.00%
Effective Rate	8.51%	[2]
Mortgage notes payable | 500 Mamaroneck Avenue [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	32,076		32,620
Stated Rate	5.41%
Effective Rate	6.47%	[2]
Mortgage notes payable | 250 West 57th Street - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	24,976		25,621
Stated Rate	5.33%
Effective Rate	6.46%	[2]
Mortgage notes payable | 250 West 57th Street - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	11,036		11,252
Stated Rate	6.13%
Effective Rate	7.26%	[2]
Mortgage notes payable | Metro Center Debt [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	94,501		96,158
Stated Rate	5.89%
Effective Rate	6.76%	[2]
Mortgage notes payable | 10 Union Square [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	20,726		20,972
Stated Rate	6.00%
Effective Rate	6.76%	[2]
Mortgage notes payable | 10 Bank Street [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	32,999		33,444
Stated Rate	5.72%
Effective Rate	6.21%	[2]
Mortgage notes payable | 1542 Third Avenue [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	18,727		19,011
Stated Rate	5.90%
Effective Rate	6.58%	[2]
Mortgage notes payable | First Stamford Place [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	243,156		245,629
Stated Rate	5.65%
Effective Rate	6.17%	[2]
Mortgage notes payable | 1010 Third Avenue and 77 West 55th Street [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	27,723		28,096
Stated Rate	5.69%
Effective Rate	6.14%	[2]
Mortgage notes payable | 383 Main Avenue [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	29,993		30,403
Stated Rate	5.59%
Effective Rate	6.10%	[2]
Mortgage notes payable | 1333 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	69,798		70,447
Stated Rate	6.32%
Effective Rate	3.82%	[2]
Mortgage notes payable | 1400 Broadway - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	69,920		0
Stated Rate	6.12%
Effective Rate	3.85%
Mortgage notes payable | 1400 Broadway - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	9,853		0
Stated Rate	3.35%
Effective Rate	1.03%
Mortgage notes payable | 112 West 34th Street - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	77,744		0
Stated Rate	6.01%
Effective Rate	3.45%
Mortgage notes payable | 112 West 34th Street - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	9,792		0
Stated Rate	6.56%
Effective Rate	4.01%
Mortgage notes payable | 1350 Broadway - First Lien Mortgage Loan [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	39,033		39,420
Stated Rate	5.87%
Effective Rate	4.39%	[2]
Mortgage notes payable | Exchangeable Senior Notes [Member]
Debt Instrument [Line Items]
Fixed rate mortgage debt	250,000		0
Stated Rate	2.625%
Effective Rate	3.92495%
Mortgage notes payable | 501 Seventh Avenue - Third Lien [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	0		6,540
Mortgage notes payable | 1350 Broadway - 2nd Lien Mortgage Loan [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	13,677		13,409
Mortgage notes payable | 1350 Broadway - 2nd Lien Mortgage Loan [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Stated Rate			4.25%	[3]
Basis spread on variable rate	1.00%	[3]
Interest rate at end of period	4.25%	[3]
Mortgage notes payable | One Grand Central Place - Third Lien [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	6,382		6,382
Stated Rate	3.75%	[4]
Basis spread on variable rate	0.50%	[4]
Interest rate at end of period	3.75%	[4]
Mortgage notes payable | 250 West 57th Street - Third lien [Member]
Debt Instrument [Line Items]
Stated Rate	4.25%	[5]
Basis spread on variable rate	1.00%	[5]
Interest rate at end of period	4.25%	[5]
Principal outstanding minimum increments	5,000	[5]
Mortgage notes payable | 250 West 57th Street - Third lien [Member] | Interest Rate Fix Option 1 [Member]
Debt Instrument [Line Items]
Stated Rate	4.75%	[5]
Basis spread on variable rate	3.00%	[5]
Number of days prior to the Effective date to fix the interest rate	2 days	[5]
Mortgage notes payable | 250 West 57th Street - Third lien [Member] | Interest Rate Fix Option 2 [Member]
Debt Instrument [Line Items]
Stated Rate	5.00%	[5]
Basis spread on variable rate	3.00%	[5]
Number of days prior to the Effective date to fix the interest rate	30 days	[5]
Mortgage notes payable | 250 West 57th Street - Third lien [Member] | Prime Rate [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	21,000		21,000
Mortgage notes payable | Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	55,600		25,000
Mortgage notes payable | Secured Revolving Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.20%	[6]
Interest rate at end of period	1.36%	[6]
Mortgage notes payable | Secured Term Credit Facility [Member]
Debt Instrument [Line Items]
Floating rate mortgage debt	$ 300,000		$ 300,000
Mortgage notes payable | Secured Term Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%	[7]
Interest rate at end of period	1.51%	[7]

[1] The loan was consolidated in February 2014.

[2] The effective rate is the yield as of September 30, 2014, including the effects of debt issuance costs.

[3] Interest at the greater of 4.25% and Prime plus 1.0%. The rate at September 30, 2014 was 4.25%.

[4] Interest at the greater of Prime plus 0.50% and 3.75%. The rate as of September 30, 2014 was 3.75%.

[5] Interest at the greater of 4.25% and Prime plus 1.0%. Prior to January 5, 2015, we have the option to fix the interest rate on all or any portion of the principal then outstanding, up to three times and in minimum increments of $5,000 to an annual rate equal to either (i) the greater of (a) 4.75% or (b) 300 basis points in excess of the weekly average yield on United States Treasury Securities adjusted to a maturity closest to January 5, 2015 as most recently made available by the Federal Reserve Board as of two days prior to the effective date of the fixing of the interest rate, and (ii) the greater of (a) 5.00% or (b) 300 basis points in excess of the weekly average yield on United States Treasury Securities adjusted to a maturity closest to January 5, 2015 as most recently made available by the Federal Reserve Board as of 30 days prior to the effective date of the fixing of the interest rate. If option (i) is selected, we will be subject to the payment of pre‑payment fees, and if option (ii) is selected, we may prepay the loan without any pre‑payment fees. The rate as of September 30, 2014 was 4.25%.

[6] Floating at 30 day LIBOR plus 1.20%. The rate as of September 30, 2014 was 1.36%.

[7] Floating at 30 day LIBOR plus 1.35%. The rate at September 30, 2014 was 1.51%.