Notes Payable, Interest Expense and Commitments and Contingencies Credit Facility Info (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Less than or equal to 40% [Member]
Line of Credit Facility [Line Items]
Line of Credit, Leverage Ratio, Upper Limit	40.00%
Line of Credit Facility, Commitment Fee Percentage	0.20%
Greater than 40% but less than or equal to 50% [Member]
Line of Credit Facility [Line Items]
Line of Credit, Leverage Ratio, Upper Limit	50.00%
Line of Credit, Leverage Ratio, Lower Limit	40.00%
Line of Credit Facility, Commitment Fee Percentage	0.25%
Greater than 50% but less than or equal to 55% [Member]
Line of Credit Facility [Line Items]
Line of Credit, Leverage Ratio, Upper Limit	55.00%
Line of Credit, Leverage Ratio, Lower Limit	50.00%
Line of Credit Facility, Commitment Fee Percentage	0.35%
Greater than 55% but less than or equal to 60% [Member]
Line of Credit Facility [Line Items]
Line of Credit, Leverage Ratio, Upper Limit	60.00%
Line of Credit, Leverage Ratio, Lower Limit	55.00%
Line of Credit Facility, Commitment Fee Percentage	0.40%
Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Issuance Date	Feb. 28, 2012
Previous Borrowing Capacity Under the Credit Facility	350
Issuance date of prior credit facility	Aug. 29, 2007
Original Maturity Date of Credit Facility	August 29, 2012
Debt Instrument, Maturity Date	Feb. 28, 2016
Credit Facility, Potential Increased Capacity	150
Credit Facility, Potential Increased Borrowing Capacity	500
Unencumbered interest coverage ratio	2.00
Fixed Charges Coverage Ratio	1.40
Fixed Charges Coverage Ratio during Extension Period	1.50
Leverage Ratio Maximum under Credit Facility	60.00%
Base Rate [Member] | Less than or equal to 40% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Base Rate [Member] | Greater than 40% but less than or equal to 50% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.60%
Base Rate [Member] | Greater than 50% but less than or equal to 55% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.90%
Base Rate [Member] | Greater than 55% but less than or equal to 60% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.10%
Prime interest rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	prime rate
Federal Funds Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	Federal Funds Rate
Debt Instrument, Basis Spread on Variable Rate	0.50%
Eurodollar Rate [Member] | Less than or equal to 40% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.50%
Eurodollar Rate [Member] | Greater than 40% but less than or equal to 50% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.60%
Eurodollar Rate [Member] | Greater than 50% but less than or equal to 55% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.90%
Eurodollar Rate [Member] | Greater than 55% but less than or equal to 60% [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.10%
Eurodollar Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	Eurodollar Rate
Line of Credit Facility, Description of Variable Base Rate During Period	Eurodollar rate
Debt Instrument, Basis Spread on Variable Rate	1.00%
Eurodollar Rate [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.60%