Schedule III - Real Estate and Accumulated Depreciation (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Changes in cost of properties
Beginning balance	$ 3,811,950		$ 3,859,960	$ 4,038,932
Acquisitions of operating properties	0		0	33,684
Building and land improvements	254,868		249,639	214,418
Sales	(48,466)		(141,045)	(291,491)
Impairments	(3,042)		(45,931)	(121,557)
Other dispositions	(974)		(110,673)	(13,891)
Other	0		0	(135)
Ending balance	4,014,336	[1],[2]	3,811,950	3,859,960
Changes in accumulated depreciation
Beginning balance	597,649		568,176	577,601
Depreciation expense	111,326		92,677	93,158
Sales	(3,129)		(9,542)	(40,346)
Impairments	(1,671)		(14,863)	(58,855)
Other dispositions	(1,092)		(38,799)	(3,247)
Other	0		0	(135)
Ending balance	$ 703,083	[3]	$ 597,649	$ 568,176

[1] As discussed in Note 3 to our Consolidated Financial Statements, we recognized impairment losses of $1.4 million primarily in connection with certain of our operating properties in the Greater Baltimore region that were disposed in the current period.

[2] The aggregate cost of these assets for Federal income tax purposes was approximately $3.4 billion at December 31, 2014.

[3] The estimated lives over which depreciation is recognized follow: Building and land improvements: 10-40 years; and tenant improvements: related lease terms.