Deferred Leasing Costs and Acquisition-related Intangible Assets and Liabilities, net (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Deferred Leasing Costs and Acquisition-related Intangible Assets, Net
Total deferred leasing costs and acquisition-related intangible assets, net	$ 187,307	[1]	$ 155,522
Acquisition-related Intangible Liabilities, Net
Total acquisitions-related intangible liabilities, net	62,275	[1],[2]	36,587	[2]
Deferred Leasing Costs [Member]
Deferred Leasing Costs and Acquisition-related Intangible Assets, Net
Finite-lived intangible assets, gross	153,288		142,652
Finite-lived intangible assets accumulated amortization	(55,641)		(52,974)
Finite-lived intangible assets, net	97,647	[1]	89,678
Above-Market Leases [Member]
Deferred Leasing Costs and Acquisition-related Intangible Assets, Net
Finite-lived intangible assets, gross	27,927		28,143
Finite-lived intangible assets accumulated amortization	(10,691)		(8,101)
Finite-lived intangible assets, net	17,236	[3]	20,042
In-Place Leases [Member]
Deferred Leasing Costs and Acquisition-related Intangible Assets, Net
Finite-lived intangible assets, gross	98,788		61,355
Finite-lived intangible assets accumulated amortization	(26,901)		(15,753)
Finite-lived intangible assets, net	71,887	[1]	45,602
Below Market Ground Lease Obligation [Member]
Deferred Leasing Costs and Acquisition-related Intangible Assets, Net
Finite-lived intangible assets, gross	690		200
Finite-lived intangible assets accumulated amortization	(153)		0
Finite-lived intangible assets, net	537	[4]	200
Below Market Leases [Member]
Acquisition-related Intangible Liabilities, Net
FiniteLivedIntangibleLiabilitiesGross	70,036		37,582
Accumulated amortization	(13,984)		(6,158)
Acquisition-related Intangible Liabilities, net	56,052	[1],[2],[5]	31,424	[2]
Above-Market Ground Lease Obligation [Member]
Acquisition-related Intangible Liabilities, Net
FiniteLivedIntangibleLiabilitiesGross	6,320		5,200
Accumulated amortization	(97)		(37)
Acquisition-related Intangible Liabilities, net	$ 6,223	[2],[6]	$ 5,163	[2]

[1] Balances and accumulated amortization amounts at September 30, 2012 reflect the write-off of the following fully amortized amounts at January 1, 2012: deferred leasing costs (approximately $9.5 million), above-market leases (approximately $1.6 million), in-place leases (approximately $3.7 million), and below-market leases (approximately $1.0 million).

[2] Included in deferred revenue and acquisition-related intangible liabilities, net in the consolidated balance sheets.

[3] Represents estimated annual amortization related to above-market operating leases. Amounts will be recorded as a decrease to rental income in the consolidated statements of operations

[4] Represents estimated annual amortization related to below−market ground lease obligations. Amounts will be recorded as an increase to ground lease expense in the consolidated statements of operations.

[5] Represents estimated annual amortization related to below-market operating leases. Amounts will be recorded as an increase to rental income in the consolidated statements of operations.

[6] Represents estimated annual amortization related to above−market ground lease obligations. Amounts will be recorded as a decrease to ground lease expense in the consolidated statements of operations.