Activity for Real Estate Investments and Accumulated Depreciation (Details) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Real Estate Investments [Abstract]
Balance at beginning of year	$ 3,379,929,000		$ 2,941,261,000		$ 2,698,682,000
Acquisitions	344,869,000	[1]	362,745,000	[1]	278,810,000	[1]
Less: FMV of Leases included in Acquisitions	(3,144,000)		(3,829,000)		(1,764,000)
Improvement and development	134,118,000		104,705,000		57,331,000
Disposition of real estate assets	(126,066,000)	[2]	(24,953,000)	[2]	(91,798,000)	[2]
Balance at end of year	3,729,706,000		3,379,929,000		2,941,261,000
Real Estate and Accumulated Depreciation Disclosure [Abstract]
Balance at beginning of year	973,126,000		873,903,000		773,438,000
Depreciation	124,691,000		112,726,000		101,454,000
Disposition of real estate assets	(57,344,000)	[2]	(13,503,000)	[2]	(989,000)	[2]
Balance at end of year	1,040,473,000		973,126,000		873,903,000
Accumulated depreciation	1,027,618,000		961,724,000
Fair value of debt assumed	32,000,000
Commercial Real Estate [Member]
Real Estate and Accumulated Depreciation Disclosure [Abstract]
Balance at end of year	12,855,000
Accumulated depreciation	$ 12,855,000		$ 11,402,000		$ 9,967,000

[1] Includes $32 million of non-cash activity related to fair market value of debt assumed.

[2] Includes assets sold, casualty losses, and removal of certain fully depreciated assets.