Fair Value Measurements and Disclosures (Details 1) (USD $)	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2013 Carrying Value [Member]	Mar. 31, 2012 Carrying Value [Member]	Mar. 31, 2013 Fair Value [Member]		Mar. 31, 2012 Fair Value [Member]		Mar. 31, 2013 Fair Value (Level 2) [Member] Carrying Value [Member]	Mar. 31, 2012 Fair Value (Level 2) [Member] Carrying Value [Member]	Mar. 31, 2013 Fair Value (Level 2) [Member] Fair Value [Member]	Mar. 31, 2012 Fair Value (Level 2) [Member] Fair Value [Member]	Mar. 31, 2013 Unsecured Debt [Member] Fair Value (Level 1) [Member] Carrying Value [Member]	Mar. 31, 2012 Unsecured Debt [Member] Fair Value (Level 1) [Member] Carrying Value [Member]	Mar. 31, 2013 Unsecured Debt [Member] Fair Value (Level 1) [Member] Fair Value [Member]	Mar. 31, 2012 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															$ 873,300,000	$ 573,300,000	$ 960,100,000	$ 653,000,000
Liabilities
Secured Debt	570,676,000	[1]	561,096,000	[1]	570,676,000	561,096,000	603,791,000	[2]	591,993,000	[2]
Exchangeable senior notes, net	165,022,000		163,944,000		165,022,000	163,944,000	180,612,000	[2]	181,223,000	[2]
Unsecured debt	1,430,880,000		1,130,895,000		1,430,880,000	1,130,895,000	1,561,378,000	[3]	1,254,047,000	[3]	557,600,000	557,600,000	601,300,000	601,000,000
Unsecured line of credit	$ 0		$ 185,000,000		$ 0	$ 185,000,000	$ 0	[2]	$ 185,049,000	[2]

[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 $873.3 million and $960.1 million, respectively, as of March 31, 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 $557.6 million and $601.3 million, respectively, as of March 31, 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.