Borrowing Arrangements (Details 1) (USD $)	12 Months Ended					1 Months Ended		1 Months Ended			12 Months Ended	1 Months Ended		12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Eurocurrency rate loans	Dec. 31, 2012 Base rate loans	Dec. 31, 2011 Line of Credit	Dec. 31, 2012 Line of Credit	Oct. 31, 2012 Unsecured Line of Credit [Member]	Aug. 31, 2012 Unsecured Line of Credit [Member]	Dec. 31, 2011 Unsecured Line of Credit [Member]	Dec. 31, 2012 Unsecured Line of Credit [Member]	Dec. 31, 2011 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Dec. 31, 2012 Federal funds rate Base rate loans	Dec. 31, 2012 LIBOR Base rate loans	Dec. 31, 2012 LIBOR Senior unsecured term loan	Dec. 31, 2012 Minimum Unsecured Line of Credit [Member]	Dec. 31, 2012 Maximum Unsecured Line of Credit [Member]
Debt Instrument [Line Items]
Interest rate (as a percent)	9.00%		7.125%	1.10%	0.10%
Facility fee (as a percent)											0.18%						0.15%	0.45%
Current borrowing capacity											$ 2,000,000,000
Variable interest rate								LIBOR				LIBOR		federal funds rate	LIBOR
Basis spread on variable rate (as a percent)										1.25%	1.20%			0.50%	1.00%	1.20%
Maximum borrowing capacity											2,500,000,000
Maximum available for issuance of letters of credit											200,000,000
Maximum available for issuance of swingline loans											200,000,000
Maximum available for issuance of loans in certain alternative currencies											250,000,000
Percentage of line of credit facility for negotiated rate loans											50.00%
Line of credit facility optional extension period (in years)	1 year
Debt amount, prepaid in full									200,000,000
Fixed interest rate (as a percent)									4.00%
Amount outstanding							540,700,000						500,000,000
Letters of credit outstanding							5,900,000
Remaining borrowing capacity							1,450,000,000
Loss on extinguishment of debt	(37,640,000)	(27,604,000)	(9,791,000)			2,400,000
Principal amount of debt issued								180,000,000		500,000,000
Weighted average maturity on unsecured term loan (in years)										4 years 6 months
Length of tranche										3 years
Length of tranche										5 years
Option to increase the borrowing capacity subject to the satisfaction of certain conditions													$ 900,000,000