Real Estate (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Allocation of the purchase price for One Beach Street and City Center Bellevue
Land	$ 40,467
Building	202,417
Land improvements	184
Tenant improvements	6,414
Total real estate	249,482
Lease intangibles	16,011
Prepaid expenses and other assets	2,597
TOTAL ASSETS	1,860,937	1,709,281
Accounts payable and accrued expenses	33,950	25,476
Security deposits payable	5,482	4,790
Lease intangibles	10,115
Other liabilities and deferred credits	61,140	55,808
Total liabilities	1,205,640	1,029,553
Beach Street [Member]
Allocation of the purchase price for One Beach Street and City Center Bellevue
Land	15,332
Building	16,764
Land improvements	30
Tenant improvements	1,223
Total real estate	33,349
Lease intangibles	4,141
Prepaid expenses and other assets	1
TOTAL ASSETS	37,491
Accounts payable and accrued expenses	94
Security deposits payable	75
Lease intangibles	1,382
Other liabilities and deferred credits	22
Total liabilities	1,573
City Center Bellevue [Member]
Allocation of the purchase price for One Beach Street and City Center Bellevue
Land	25,135
Building	185,653
Land improvements	154
Tenant improvements	5,191
Total real estate	216,133
Lease intangibles	11,870
Prepaid expenses and other assets	2,596
TOTAL ASSETS	230,599
Accounts payable and accrued expenses	456
Security deposits payable	740
Lease intangibles	8,733
Other liabilities and deferred credits	497
Total liabilities	$ 10,426