Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair Value of Financial Instruments
Fair value of notes payable	$ 750,400,000	$ 697,300,000
Carrying value on outstanding balance of notes payable	$ 757,135,000	$ 696,988,000

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents notes payable as of the balance sheet date.

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