Debt Lines of Credit (Details) (USD $)	12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012 A 175 billion LOC [Member]	Dec. 31, 2013 A 175 billion LOC [Member]	Dec. 31, 2013 A 250 billion LOC [Member] [Domain]	Dec. 31, 2012 $1.25 billion LOC [Member]	Dec. 31, 2011 $1.25 billion LOC [Member]	Dec. 31, 2011 $1.425 Billion LOC [Member]	Dec. 31, 2010 $1.425 Billion LOC [Member]	Dec. 31, 2013 Revolving Credit Facility [Member]	Dec. 31, 2013 London Interbank Offered Rate (LIBOR) [Member]	Dec. 31, 2012 London Interbank Offered Rate (LIBOR) [Member]	Dec. 31, 2013 Line of Credit [Member] London Interbank Offered Rate (LIBOR) [Member]	Dec. 31, 2012 Line of Credit [Member] London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Unsecured revolving credit facility				$ 1,750,000,000	$ 2,500,000,000	$ 1,750,000,000	$ 1,250,000,000		$ 1,425,000,000
Debt Instrument, Maturity Date	Sep. 01, 2014				Apr. 01, 2018
500.0 million ability to increase	500,000,000						500,000,000
Debt Instrument, Description of Variable Rate Basis											LIBOR plus 1.75%	LIBOR plus 0.50%	LIBOR plus 1.05%	LIBOR plus 1.15%
Line Of Credit Facility Commitment Fee													15 Basis Points
Advances under credit facility, rate plus spread						1.15%				1.05%
Line of Credit Maturity							Jul. 13, 2014		Feb. 01, 2012
Increase to Line of Credit						500,000,000
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		2,350,000,000
Amount restricted/dedicated to support letters of credit			30,200,000		34,900,000
Long-term Line of Credit	$ 115,000,000	$ 0
Weighted average interest rate on revolving letter of credit			1.35%		1.26%