Notes Payable and Debt and Financing Arrangements (Details) - Narrative relating to credit facilities (USD $)	12 Months Ended		12 Months Ended					0 Months Ended					0 Months Ended	12 Months Ended
	Dec. 31, 2011 LOCs	Dec. 31, 2011 Outstanding letters of credit and revolving credit loans	Dec. 31, 2011 2007 facility	Dec. 31, 2012 March 2011 Credit agreement	Dec. 31, 2012 December 2011 Secured credit agreement	Dec. 31, 2011 December 2011 Secured credit agreement	Dec. 31, 2011 December 2011 Unsecured credit agreement	Jun. 22, 2010 Letter of credit facility	Dec. 31, 2012 Letter of credit facility	Dec. 31, 2011 Letter of credit facility	Oct. 31, 2011 Letter of credit facility Letter of credit facility of subsidiary	Mar. 25, 2011 Letter of credit facility March 2011 Credit agreement	Aug. 03, 2010 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Dec. 31, 2011 Revolving credit facility	Dec. 31, 2011 Revolving credit facility December 2011 Unsecured credit agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use									$ 1,757,250,000	$ 1,871,192,000
Available credit facilities - commitments	3,000,000,000		4,000,000,000			650,000,000	1,350,000,000		4,000,000,000	4,000,000,000		1,000,000,000		1,000,000,000	1,000,000,000	1,000,000,000
Line Of Credit Facility Optional Increased Capacity				500,000,000	500,000,000
Number of credit facilities entered into	3
Term of credit facility			5 years					5 years					5 years	4 years
Terminated credit capacity		3,000,000,000	750,000,000					2,300,000,000			75,000
Terminated revolving credit capacity													$ 100,000,000