Debt Lines of Credit (Details) (USD $)	2 Months Ended				12 Months Ended		12 Months Ended
	Feb. 21, 2013	Feb. 15, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 $1.25 billion LOC [Member]	Dec. 31, 2011 $1.25 billion LOC [Member]	Dec. 31, 2012 $1.425 Billion LOC [Member]	Dec. 31, 2011 $1.425 Billion LOC [Member]	Dec. 31, 2010 $1.425 Billion LOC [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility		$ 2,500,000,000	$ 1,750,000,000		$ 1,750,000,000	$ 1,250,000,000			$ 1,425,000,000
Line of Credit Maturity					Jul. 13, 2014		Feb. 01, 2012
500.0 million ability to increase	500,000,000				500,000,000
Increase to Line of Credit					500,000,000
Advances under credit facility, rate plus spread					1.15%
Line of Credit Facility, Commitment Fee Percentage					0.20%
Write Off Of Unamortized Deferred Financing Costs								200,000
Remaining borrowing capacity					1,720,000,000			1,220,000,000
Line of Credit Facility, Amount Outstanding			0	0
Amount restricted/dedicated to support letters of credit					$ 30,200,000			$ 31,800,000
Weighted average interest rate on revolving letter of credit					1.35%			1.42%