Fair Value Measurements and Disclosures (Details 1) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Liabilities
Secured Debt	$ 561,096,000		$ 351,825,000
Exchangeable notes	163,944,000		306,892,000
Unsecured Debt	1,130,895,000		980,569,000
Credit Facility	185,000,000		182,000,000
Fair Value [Member]
Liabilities
Secured Debt	591,993,000	[1]	367,402,000	[1]
Exchangeable notes	181,223,000	[1]	320,919,000	[1]
Unsecured Debt	1,254,047,000	[2]	1,011,982,000	[2]
Credit Facility	185,049,000	[1]	182,299,000	[1]
Fair Value [Member] | Level 2 [Member]
Liabilities
Unsecured Debt	558,000,000		88,900,000
Carrying Value [Member]
Liabilities
Secured Debt	561,096,000	[1]	351,825,000	[1]
Exchangeable notes	163,944,000	[1]	306,892,000	[1]
Unsecured Debt	1,130,895,000	[2]	980,569,000	[2]
Credit Facility	185,000,000	[1]	182,000,000	[1]
Carrying Value [Member] | Level 2 [Member]
Liabilities
Unsecured Debt	601,000,000		83,000,000
Unsecured Debt [Member] | Fair Value [Member] | Level 1 [Member]
Liabilities
Debt	573,000,000		923,100,000
Unsecured Debt [Member] | Carrying Value [Member] | Level 1 [Member]
Liabilities
Debt	$ 653,000,000		$ 897,600,000

[1] 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.

[2] 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 $653.0 million and $573.0 million, respectively, as of December 31, 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 $601.0 million and $558.0 million, respectively, as of December 31, 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.