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Debt (Details) $ in Thousands			3 Months Ended		9 Months Ended		12 Months Ended
Debt (Details) $ in Thousands	Jun. 07, 2017 USD ($)	Apr. 24, 2017 USD ($) mo	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) yr	Sep. 30, 2016 USD ($)	Dec. 31, 2008 USD ($)	Jun. 06, 2017	Apr. 23, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Proceeds from mortgage notes payable					$ 2,300,000	$ 0
Interest Payable			$ 99,100		99,100					$ 243,933
Outside members’ notes payable			0		0					$ 180,000
Repayments of mortgage notes payable					1,313,890	1,323,284
Repayments of Mezzannine notes payable					306,000	0
Gains from early extinguishments of debt			$ 0	$ (371)	$ 14,354	$ (371)
767 Fifth Avenue (the General Motors Building)
Debt Instrument [Line Items]
Gains from early extinguishments of debt	$ 14,600
767 Fifth Avenue (the General Motors Building) \| Secured Debt [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate		3.43%
767 Fifth Avenue (the General Motors Building) \| Members' notes Payables [Member]
Debt Instrument [Line Items]
Repayment of a portion of members loans accrued interest and principal balance	176,100
Consolidation, Eliminations [Member] \| 767 Fifth Avenue (the General Motors Building) \| Other Mezzanine Debt [Member]
Debt Instrument [Line Items]
Repayments of Other Mezzanine Debt							$ 263,100
Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building)
Debt Instrument [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity	$ 273,900
Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building) \| Secured Debt [Member]
Debt Instrument [Line Items]
Ownership Percentage by the Company	60.00%
Proceeds from mortgage notes payable	$ 2,300,000
Coupon/Stated Rate	3.43%							5.95%
Repayments of mortgage notes payable	$ 1,300,000
Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building) \| Mezzanine Debt [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate	6.02%
Repayments of Mezzannine notes payable	$ 306,000
Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building) \| Other Mezzanine Debt [Member]
Debt Instrument [Line Items]
Other Long-term Debt	$ 294,000
Coupon/Stated Rate	6.02%
Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building) \| Members' notes Payables [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate	11.00%
Interest Payable	$ 425,000
Outside members’ notes payable	450,000
Consolidated Entities [Member] \| Consolidation, Eliminations [Member] \| 767 Fifth Avenue (the General Motors Building)
Debt Instrument [Line Items]
Unpaid principal balance of the members' notes payable contributed to equity	164,400
Consolidated Entities [Member] \| Consolidation, Eliminations [Member] \| 767 Fifth Avenue (the General Motors Building) \| Members' notes Payables [Member]
Debt Instrument [Line Items]
Interest Payable	255,000
Outside members’ notes payable	270,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Competitive bid quote		65.00%
Maximum Leverage Ratio					60.00%
Line of Credit Facility, Current Borrowing Capacity		$ 1,500,000							$ 1,000,000
Leverage Ratio Maximum Increasing Limit					65.00%
Leverage Ratio Reduction Limit					60.00%
Leverage Ratio Reduction Period, Years \| yr					1
Maximum Secured Debt Leverage Ratio					55.00%
Fixed Charge Coverage Ratio Minimum.			1.40		1.40
Maximum Unsecured Debt Leverage Ratio					60.00%
Unsecured Debt Leverage Ratio Maximum Increasing Limit					65.00%
Unsecured Debt Leverage Ratio Reduction Limit					60.00%
Unsecured Debt Leverage Ratio Reduction Period, Years \| yr					1
Unsecured Debt Interest Coverage Ratio Minimum.					1.75
Revolving Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.10%
Revolving Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.30%
Delayed Draw Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage		0.15%
Line of Credit Facility, Frequency of Commitment Fee Payment		4
minimum borrowing amount		$ 50,000
Line of Credit Facility, Current Borrowing Capacity		$ 500,000
Alternative Base Interest Rate Calculation [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Amount added to Federal Funds Rate to Calculate Interest Rate		0.50%
LIBOR Period Used to Calculate Interest Rate (in months) \| mo		1
Amount added to LIBOR to Calculate Interest Rate		1.00%
Alternative Base Interest Rate Calculation [Member] \| Revolving Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.00%
Alternative Base Interest Rate Calculation [Member] \| Revolving Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.55%
Alternative Base Interest Rate Calculation [Member] \| Delayed Draw Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.00%
Alternative Base Interest Rate Calculation [Member] \| Delayed Draw Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.75%
Interest Rate Based on LIBOR or CDOR [Member] \| Revolving Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.775%
Interest Rate Based on LIBOR or CDOR [Member] \| Revolving Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		1.55%
Interest Rate Based on LIBOR or CDOR [Member] \| Delayed Draw Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		0.85%
Interest Rate Based on LIBOR or CDOR [Member] \| Delayed Draw Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate		1.75%
Current credit rating [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage					0.15%
Current credit rating [Member] \| Alternative Base Interest Rate Calculation [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate			0.00%		0.00%
Current credit rating [Member] \| Alternative Base Interest Rate Calculation [Member] \| Delayed Draw Facility [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate			0.00%		0.00%
Current credit rating [Member] \| Interest Rate Based on LIBOR or CDOR [Member] \| Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate			0.875%		0.875%
Current credit rating [Member] \| Interest Rate Based on LIBOR or CDOR [Member] \| Delayed Draw Facility [Member]
Debt Instrument [Line Items]
Margin added to Calculated Interest Rate			0.95%		0.95%
Parent Company [Member] \| Consolidated Entities [Member] \| 767 Fifth Avenue (the General Motors Building) \| Members' notes Payables [Member]
Debt Instrument [Line Items]
Interest Payable	170,000
Outside members’ notes payable	$ 180,000