Fair Value Measurements and Disclosures (Details 1) (USD $)	Mar. 31, 2014		Dec. 31, 2013
Liabilities
Secured Debt	$ 556,946,000	[1]	$ 560,434,000	[1]
Exchangeable senior notes, net	169,528,000		168,372,000
Unsecured debt	1,431,217,000		1,431,132,000
Unsecured line of credit	0		45,000,000
Carrying Value [Member]
Liabilities
Secured Debt	556,946,000		560,434,000
Exchangeable senior notes, net	169,528,000		168,372,000
Unsecured debt	1,431,217,000		1,431,132,000
Unsecured line of credit	0		45,000,000
Fair Value [Member]
Liabilities
Secured Debt	567,107,000	[2]	568,760,000	[2]
Exchangeable senior notes, net	175,403,000	[2]	178,190,000	[2]
Unsecured debt	1,506,815,000	[3]	1,523,052,000	[3]
Unsecured line of credit	0	[2]	45,012,000	[2]
Fair Value (Level 2) [Member] | Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	557,700,000		557,700,000
Fair Value (Level 2) [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	585,600,000		593,700,000
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	873,500,000		873,500,000
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	$ 921,200,000		$ 929,300,000

[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 $873.5 million and $921.2 million, respectively, as of March 31, 2014. The carrying value and fair value of the Level I instruments as of December 31, 2013, was $873.5 million and $929.3 million, respectively. The carrying value and fair value of the Level II instruments was $557.7 million and $585.6 million, respectively, as of March 31, 2014. The carrying value and fair value of the Level II instruments as of December 31, 2013, was $557.7 million and $593.7 million, respectively.