Notes Payable and Debt and Financing Arrangements (Details) - Financing Structure (USD $)			0 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2012 LOCs	Dec. 31, 2011	Jun. 22, 2010 Letter of credit facility	Dec. 31, 2012 Letter of credit facility	Dec. 31, 2011 Letter of credit facility	Aug. 03, 2010 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Dec. 31, 2011 Revolving credit facility	Dec. 31, 2012 Senior notes 6.5% Guaranteed Senior Notes due 2012	Dec. 31, 2011 Senior notes 6.5% Guaranteed Senior Notes due 2012	Dec. 31, 2012 Senior notes 5.25% Senior Notes due 2014	Dec. 31, 2011 Senior notes 5.25% Senior Notes due 2014	Dec. 31, 2012 Senior notes 5.75% Senior Notes due 2021	Dec. 31, 2011 Senior notes 5.75% Senior Notes due 2021	Dec. 31, 2012 Senior notes 6.375% Senior Notes due 2024	Dec. 31, 2011 Senior notes 6.375% Senior Notes due 2024	Dec. 31, 2012 Senior notes 6.25% Senior Notes due 2027	Dec. 31, 2011 Senior notes 6.25% Senior Notes due 2027
Debt Instrument [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	4,000,000,000		1,000,000,000	1,000,000,000
Debt instrument face amount										600,000,000	600,000,000	600,000,000	400,000,000	400,000,000	350,000,000	350,000,000	325,000,000	325,000,000
Total commitment available	2,675,000,000	3,275,000,000
Total debt outstanding before fair value hedge	1,666,103,000	2,264,618,000								599,971,000	598,424,000	597,501,000	396,230,000	395,963,000	348,701,000	348,592,000	322,748,000	322,591,000
Adjustment to carrying value - impact of fair value hedges	6,675,000	10,709,000
Notes payable and debt	1,672,778,000	2,275,327,000
Debt instrument interest rate, stated percentage									6.50%		5.25%		5.75%		6.375%		6.25%
Number of letter of credit facilities available	5
Amount of sublimit of revolving credit facility							$ 1,000,000,000
Term of credit facility			5 years			5 years	4 years