Notes Payable and Long-term Debt (Narrative) (Details) (USD $)
3 Months Ended 12 Months Ended 60 Months Ended
Dec. 31, 2012
lenders
Dec. 31, 2011
Dec. 31, 2012
lenders
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2017
Dec. 31, 2011
Credit Facility Maturing In 2016 [Member]
Dec. 31, 2011
Notes due 2012, 5.50% [Member]
Nov. 30, 2010
Aggregate Amount On Principal Amount Due On Two Thousand Twelve [Member]
Dec. 31, 2012
$1.1B Fixed rate debt [Member]
Dec. 31, 2011
$1.0B FIxed Rate [Member]
Dec. 31, 2012
Maximum [Member]
Proceeds received from the issuance of debt $ 1,092,000,000 $ 992,000,000                    
Debt Instrument, Face Amount                   1,100,000,000 1,000,000,000  
Proceeds to exercise rights to call and repurchase long-term debt     970,000,000 0 678,000,000              
Gains (Losses) on Extinguishment of Debt     (29,000,000)                  
Extinguishment of Debt, Gain (Loss), Net of Tax     (19,000,000)                  
Number of highly rated lenders 25   25                  
Amount of new revolving credit facility             1,400,000,000 1,000,000,000 500,000,000      
Debt Instrument, Basis Spread on Variable Rate 0.90%   0.90%                  
Outstanding letters of credit that effectively reduced our borrowing capacity under credit facility 2,000,000 3,000,000 2,000,000 3,000,000                
Amounts outstanding under lines of credit   0.00   0.00                
Aggregate Maximum Percentage Of Any Single Lender                       10.00%
Principal payments due on Long Term Debt for the next 5 years           0.00            
Uncommitted bank lines             1,400,000,000 1,000,000,000 500,000,000      
Maximum total debt to capitalization percentage allowed under credit facility 60.00%   60.00%                  
Total debt to capitalization ratio 36.60%   36.60%                  
Total payments for interest on notes payable     $ 198,000,000 $ 167,000,000 $ 134,000,000