Unsecured Line of Credit (Details) (USD $)	12 Months Ended		6 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2012 After the Amendment [Member]	Jun. 23, 2011 Prior to Amendment [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 750,000,000	$ 1,000,000,000
Debt Instrument, Description of Variable Rate Basis		Eurodollar plus 1.225% per annum	Eurodollar plus 0.475%
Line of Credit Facility, Maximum Borrowing Capacity	1,000,000,000
Line of Credit Facility, Expiration Date		Jun. 24, 2014	Aug. 03, 2011
Line of Credit Maturity Extension Option, Years	1
Line Of Credit Facility Extension Fee	0.20%
Debt Instrument, Basis Spread on Variable Rate		1.225%	0.475%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.225%
Frequency of Commitment Fee Payments.	quarterly
Line of Credit Facility, Amount Outstanding	0.0
Leverage Ratio, Maximum	60.00%
Leverage Ratio Maximum Increasing Limit	65.00%
Leverage Ratio Reduction Limit	60.00%
Leverage Ratio Reduction Period, Years	1
Maximum Secured Debt Leverage Ratio	55.00%
Fixed Charge Coverage Ratio Minimum.	1.40
Maximum Unsecured Debt Leverage Ratio	60.00%
Unsecured Debt Leverage Ratio Maximum Increasing Limit	65.00%
Unsecured Debt Leverage Ratio Reduction Limit	60.00%
Unsecured Debt Leverage Ratio Reduction Period, Years	1
Minimum Net Worth Requirement	$ 3,500,000,000
Unsecured Debt Interest Coverage Ratio Minimum.	1.75