Fair Value Measurements and Disclosures (Details 1) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Liabilities
Secured Debt	$ 563,898,000	[1]	$ 561,096,000	[1]
Exchangeable senior notes, net	167,236,000		163,944,000
Unsecured debt	1,431,048,000		1,130,895,000
Unsecured line of credit	0		185,000,000
Carrying Value [Member]
Liabilities
Secured Debt	563,898,000		561,096,000
Exchangeable senior notes, net	167,236,000		163,944,000
Unsecured debt	1,431,048,000		1,130,895,000
Unsecured line of credit	0		185,000,000
Fair Value [Member]
Liabilities
Secured Debt	574,661,000	[2]	591,993,000	[2]
Exchangeable senior notes, net	178,379,000	[2]	181,223,000	[2]
Unsecured debt	1,522,228,000	[3]	1,254,047,000	[3]
Unsecured line of credit	0	[2]	185,049,000	[2]
Fair Value (Level 2) [Member] | Carrying Value [Member]
Liabilities
Unsecured debt	589,200,000		557,600,000
Fair Value (Level 2) [Member] | Fair Value [Member]
Liabilities
Unsecured debt	590,000,000		601,000,000
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	841,900,000		573,300,000
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	$ 932,200,000		$ 653,000,000

[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 using Level I and Level II inputs. Level I inputs are based on quoted prices for identical instruments in active markets. The carrying value and fair value of the Level I instruments was $841.9 million and $932.2 million, respectively, as of September 30, 2013. The carrying value and fair value of the Level I instruments at December 31, 2012, was $573.3 million and $653.0 million, respectively. The carrying value and fair value of the Level II instruments was $589.2 million and $590.0 million, respectively, as of September 30, 2013. The carrying value and fair value of the Level II instruments at December 31, 2012, was $557.6 million and $601.0 million, respectively.