Subsequent Events - Additional Information (Details)				9 Months Ended
	Oct. 07, 2016 USD ($) ft²	Oct. 06, 2016 USD ($) ft²	May 03, 2016 USD ($)	Sep. 30, 2016 USD ($) ft²	Sep. 30, 2015 USD ($)
Subsequent Event [Line Items]
Area of office properties | ft²				10,400,000
Notes and mortgages payable, interest rate				4.60%
Repayment of mortgage loan				$ 414,564,000	$ 1,024,000
Payment of swap breakage costs			$ 21,972,000
1301 Avenue of Americas [Member]
Subsequent Event [Line Items]
Maturity date of debt				2021-10
1301 Avenue of Americas [Member] | London Interbank Offered Rate (LIBOR) | Tranche B [Member]
Subsequent Event [Line Items]
Debt instrument basis spread on variable rate, description				$350,000,000 tranche at a floating rate of LIBOR plus 180 basis points (2.36% at closing).
900 Third Avenue [Member]
Subsequent Event [Line Items]
Maturity date of debt				2017-11
Notes and mortgages payable, interest rate				4.26%
900 Third Avenue [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Subsequent Event [Line Items]
Maturity date of debt				2017-11
Notes and mortgages payable, interest rate				5.98%
Waterview [Member]
Subsequent Event [Line Items]
Maturity date of debt				2017-06
Waterview [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Subsequent Event [Line Items]
Maturity date of debt				2017-06
Notes and mortgages payable, interest rate				5.76%
Subsequent Event [Member] | 1301 Avenue of Americas [Member]
Subsequent Event [Line Items]
Mortgage financing amount		$ 850,000,000
Area of office properties | ft²		1,800,000
Debt instrument term of notes		5 years
Notes and mortgages payable, interest rate		2.77%
Proceeds after repayment of existing loan		$ 827,187,000
Subsequent Event [Member] | 1301 Avenue of Americas [Member] | Tranche A [Member]
Subsequent Event [Line Items]
Mortgage financing amount		500,000,000
Subsequent Event [Member] | 1301 Avenue of Americas [Member] | Tranche B [Member]
Subsequent Event [Line Items]
Mortgage financing amount		$ 350,000,000
Subsequent Event [Member] | 1301 Avenue of Americas [Member] | London Interbank Offered Rate (LIBOR) | Tranche B [Member]
Subsequent Event [Line Items]
Notes and mortgages payable, interest rate		2.36%
Debt instrument basis spread on variable rate		1.80%
Subsequent Event [Member] | 1301 Avenue of Americas [Member] | Mortgages and Notes Payable with Fixed Rate [Member] | Tranche A [Member]
Subsequent Event [Line Items]
Notes and mortgages payable, interest rate		3.05%
Subsequent Event [Member] | 900 Third Avenue [Member]
Subsequent Event [Line Items]
Area of office properties | ft²		596,270
Notes and mortgages payable, interest rate		4.26%
Repayment of mortgage loan		$ 274,337,000
Payment of swap breakage costs		$ 7,729,000
Subsequent Event [Member] | Waterview [Member]
Subsequent Event [Line Items]
Area of office properties | ft²	647,243
Repayment of mortgage loan	$ 210,000,000
Defeasance costs incurred with relation to repayment	$ 4,734,000
Subsequent Event [Member] | Waterview [Member] | Mortgages and Notes Payable with Fixed Rate [Member]
Subsequent Event [Line Items]
Notes and mortgages payable, interest rate	5.76%