Discontinued Operations (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations									$ 4,207	$ 43,838	$ 19,841
Revenue:
Rents									461,387	437,560	441,329
Tenant Reimbursements									79,087	77,060	77,404
Termination Fees									4,497	3,233	2,951
Other									4,186	5,710	5,348
Total revenue	138,658	143,354	140,644	139,554	136,395	134,238	131,530	133,516	562,210	535,679	538,568
Expenses:
Property operating expenses									160,406	152,319	159,768
Real estate taxes									55,612	53,402	51,433
Depreciation and amortization									197,021	188,382	202,933
Total operating expenses									446,418	424,643	444,326
Investment Income, Interest									1,044	3,008	1,810
Income from discontinued operations before gain on sale of interests in real estate									825	9,064	12,794
Net gain (loss) on disposition of discontinued operations									3,382	34,774	7,047
Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income from discontinued operations									4,207	43,838	19,841
Revenue:
Rents									4,754	28,678	43,050
Tenant Reimbursements									355	2,353	5,030
Termination Fees									0	11	75
Other									123	378	110
Total revenue									5,232	31,420	48,265
Expenses:
Property operating expenses									1,839	8,687	13,940
Real estate taxes									649	3,468	5,356
Depreciation and amortization									1,921	10,208	16,178
Total operating expenses									4,409	22,363	35,474
Investment Income, Interest									2	7	3
Income from discontinued operations before gain on sale of interests in real estate									825	9,064	12,794
Net gain (loss) on disposition of discontinued operations									$ 3,382	$ 34,774	$ 7,047
Net Rentable Space Sold	2,300,000								2,300,000
Number of properties sold (in properties)										32