Real Estate and Accumulated Depreciation (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Change in Accumulated Depreciation
Balance, Beginning of Year	$ 663,310	$ 597,461	$ 524,865
Depreciation for Year	95,931	104,175	112,241
Disposition of Assets	(30,587)	(5,354)	(9,633)
Write-off of Fully Depreciated Assets	(32,723)	(32,972)	(30,012)
Balance, End of Year	$ 695,931	$ 663,310	$ 597,461