Stockholders' Equity (Details) (USD $) In Thousands, except Per Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			$ 842,740
Issuance of common stock			224,084
Common stock redemption			(51)
Stock-based compensation			1,602
Net income (loss)	2,011	6,514	(3,751)	11,173
Other comprehensive loss
Comprehensive loss			(3,751)
Dividends to common stockholders ($0.60 per share)			(42,570)
Distributions to noncontrolling interests			(247)
Proceeds from noncontrolling interests			76
Redemption of noncontrolling interest in consolidated joint ventures			(5,055)
Ending Balance	1,016,828		1,016,828
Stockholders Equity (Textuals) [Abstract]
Dividends per share to common stockholders	$ 0.30	$ 0.30	$ 0.60	$ 0.60
Common Stock [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			661
Issuance of common stock			103
Stock-based compensation			1
Other comprehensive loss
Ending Balance	765		765
Additional Paid-In Capital [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			1,641,379
Issuance of common stock			223,981
Common stock redemption			(51)
Stock-based compensation			1,601
Other comprehensive loss
Redemption of noncontrolling interest in consolidated joint ventures			(1,469)
Ending Balance	1,865,441		1,865,441
Accumulated Other Comprehensive Loss [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			(5,269)
Other comprehensive loss
Ending Balance	(5,269)		(5,269)
Cumulative Net Income [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			796,165
Net income (loss)			(3,778)
Other comprehensive loss
Ending Balance	792,387		792,387
Cumulative Dividends [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			(1,593,926)
Other comprehensive loss
Dividends to common stockholders ($0.60 per share)			(42,570)
Ending Balance	(1,636,496)		(1,636,496)
Stockholders Equity (Textuals) [Abstract]
Dividends per share to common stockholders			$ 0.60
Total Stockholders' Equity [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			839,010
Issuance of common stock			224,084
Common stock redemption			(51)
Stock-based compensation			1,602
Net income (loss)			(3,778)
Other comprehensive loss
Dividends to common stockholders ($0.60 per share)			(42,570)
Redemption of noncontrolling interest in consolidated joint ventures			(1,469)
Ending Balance	1,016,828		1,016,828
Stockholders Equity (Textuals) [Abstract]
Dividends per share to common stockholders			$ 0.60
Non-controlling Interests [Member]
Reconciliation of equity attributable to the Company and to noncontrolling interests Reconciliation of equity attributable to the Company and to noncontrolling interests
Beginning Balance			3,730
Net income (loss)			27
Other comprehensive loss
Distributions to noncontrolling interests			(247)
Proceeds from noncontrolling interests			76
Redemption of noncontrolling interest in consolidated joint ventures			(3,586)
Ending Balance	$ 0		$ 0