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Notes Payable (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended 3 Months Ended
Jun. 30, 2011
Dec. 31, 2011
years
Dec. 31, 2010
Dec. 31, 2009
Sep. 30, 2011
Line Of Credit [Member]
Dec. 31, 2011
Line Of Credit [Member]
Dec. 31, 2011
Letter Of Credit [Member]
Jun. 30, 2011
4.625% Notes Due 2021 [Member]
Dec. 31, 2011
4.625% Notes Due 2021 [Member]
Jun. 30, 2011
4.875 Notes Due 2023 [Member]
Dec. 31, 2011
4.875 Notes Due 2023 [Member]
Sep. 30, 2011
Line Of Credit [Member]
Dec. 31, 2011
Line Of Credit [Member]
Jun. 30, 2011
Commercial Banks [Member]
Term Loan, Due 2012 [Member]
Jun. 30, 2011
Senior Unsecured Notes [Member]
4.625% Notes Due 2021 [Member]
Dec. 31, 2011
Senior Unsecured Notes [Member]
4.70% Notes Due 2021 [Member]
Jun. 30, 2011
Senior Unsecured Notes [Member]
4.875 Notes Due 2023 [Member]
Dec. 31, 2011
Senior Unsecured Notes [Member]
5.00% Notes Due 2023 [Member]
Dec. 31, 2011
Senior Unsecured Notes [Member]
7.69% Notes Due 2011 [Member]
Dec. 31, 2011
Medium-Term Notes [Member]
4.99% Notes Due 2011 [Member]
Dec. 31, 2011
Floating Rate Debt [Member]
Dec. 31, 2010
Floating Rate Debt [Member]
Debt Instrument [Line Items]                                            
Amount of current borrowing capacity under unsecured credit facility         $ 500                                  
Prior maturity period of unsecured credit facility         August 2012                                  
Current maturity period of unsecured credit facility                       September 2015                    
Optional extension period of unsecured credit facility                       September 2016                    
Maximum term of bid rate loans (days)           180                                
Lesser of amount stated or the amount available under the line of credit           250                                
Maximum borrowing capacity under unsecured credit facility             100           750                  
Outstanding letters of credit under credit facility             10.4                              
Available amount under unsecured credit facility           489.6                                
Repayment of senior unsecured notes   500                       500                
Interest rate of notes payable               4.625%   4.875%                        
Yield to maturity interest rate on notes payable               4.70%   5.00%                        
Yield to maturity interest rate on notes payable                               4.70%   5.00% 7.69% 4.99%    
Aggregate principal face amount of senior unsecured notes                             250   250          
Net proceeds from notes payable 491.8                                          
Notes payable, maturity date               Jun. 01, 2021   Jun. 01, 2023                        
Discount on notes payable face amount                 99.404%   98.878%                      
Fair value adjustment of assumed debt   33.9                                        
Threshold period for redemption of notes (days)   90                                        
Redemption price as percentage of principal plus accrued and unpaid interest if redeemed at or prior to threshold period   100.00%                                        
Decrease of interest expense as a result of amortization of fair value adjustment   0.4 1.1 2.3                                    
Write off of unamortized loan costs                           $ 0.5                
Weighted average interest rate on floating rate debt   4.80%                                     1.10% 1.30%
Weighted average maturity of indebtedness (including unsecured line of credit) (in years)   6.8