Fair Value Measurements and Disclosures (Details 1) (USD $)	Sep. 30, 2012		Dec. 31, 2011
Liabilities
Secured Debt	$ 520,867,000	[1]	$ 351,825,000	[1]
Exchangeable senior notes, net	162,885,000		306,892,000
Unsecured debt	1,130,814,000		980,569,000
Credit Facility	27,000,000		182,000,000
Carrying Value [Member]
Liabilities
Secured Debt	520,867,000		351,825,000
Exchangeable senior notes, net	162,885,000		306,892,000
Unsecured debt	1,130,814,000		980,569,000
Credit Facility	27,000,000		182,000,000
Fair Value [Member]
Liabilities
Secured Debt	547,378,000	[2]	367,402,000	[2]
Exchangeable senior notes, net	179,945,000	[2]	320,919,000	[2]
Unsecured debt	1,245,462,000	[3]	1,011,982,000	[3]
Credit Facility	27,233,000	[2]	182,299,000	[2]
Fair Value (Level 2) [Member] | Carrying Value [Member]
Liabilities
Unsecured debt	233,000,000		83,000,000
Fair Value (Level 2) [Member] | Fair Value [Member]
Liabilities
Unsecured debt	241,600,000		88,900,000
Unsecured Debt [Member] | Fair Value (Level 1) [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	897,800,000		897,600,000
Unsecured Debt [Member] | Fair Value (Level 1) [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	$ 1,000,000,000		$ 923,100,000

[1] Amounts reported include the amounts of unamortized debt premiums and discounts for the periods presented.

[2] Fair value calculated using Level II inputs which are based on model−derived valuations in which significant inputs and significant value drivers are observable in active markets.

[3] Fair value calculated primarily using Level I inputs which are based on quoted prices for identical instruments in active markets. The carrying value and fair value of the Level I instruments was $897.8 million and $1.0 billion, respectively, as of September 30, 2012. The carrying value and fair value of the Level I instruments at December 31,2011, was $897.6 million and $923.1 million, respectively. The carrying value and fair value of the Level II instruments was $233.0 million and $241.6 million, respectively, as of September 30, 2012. The carrying value and fair value of the Level II instruments at December 31,2011, was $83.0 million and $88.9 million, respectively.