Fair Value Measurements - Fair Value of Financial Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Unsecured Credit Facilities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Assumptions, Risk Free Interest Rate	1.60%	1.50%
Notes Receivable [Member] | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Assumptions, Risk Free Interest Rate	6.95%	7.11%
Notes Receivable [Member] | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Assumptions, Risk Free Interest Rate	8.05%	8.12%
Notes Payable to Banks [Member] | Minimum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Assumptions, Risk Free Interest Rate	2.40%	2.36%
Notes Payable to Banks [Member] | Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Assumptions, Risk Free Interest Rate	3.25%	4.25%
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Receivable, Fair Value	23,700,000	35,300,000
Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Notes Payable, Fair Value	2,000,000,000	2,100,000,000
Fair Value, Inputs, Level 2 [Member] | Unsecured Credit Facilities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Fair Value Disclosure	170,200,000	40,000,000