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Bank and Other Notes Payable: (Details) (USD $)
In Thousands, unless otherwise specified
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended 1 Months Ended 9 Months Ended
Mar. 31, 2007
Senior Notes
Mar. 31, 2012
Senior Notes
Sep. 30, 2012
Senior Notes
Dec. 31, 2011
Senior Notes
Sep. 30, 2012
Line of Credit
Dec. 31, 2011
Line of Credit
Apr. 25, 2011
Line of Credit
Sep. 30, 2012
Line of Credit
Low end of range
Apr. 25, 2011
Line of Credit
Low end of range
Sep. 30, 2012
Line of Credit
High end of range
Apr. 25, 2011
Line of Credit
High end of range
Dec. 31, 2011
Unsecured term loan
Sep. 30, 2012
Unsecured term loan
Dec. 08, 2011
Unsecured term loan
Low end of range
Dec. 08, 2011
Unsecured term loan
High end of range
Sep. 30, 2012
Greeley Note
Dec. 31, 2011
Greeley Note
Bank and other notes payable                                  
Debt issued $ 950,000                     $ 125,000          
Interest rate on debt (as a percent)     3.25%                            
Conversion rate per $1 principal amount   8.9702                              
Convertible debt principal amount, basis for exchange   1                              
Repayment of Senior Notes   439,318                              
Debt, carrying value       437,788                       24,239 24,848
Debt, unamortized discount       1,530                          
Term of debt                       7 years          
Fair value of debt                       120,019 123,163        
Effective interest rate (as a percent)       5.41%               2.42% 2.57%     6.34%  
Debt, fair value       439,318                       25,138 26,510
Revolving line of credit         1,500,000   1,500,000                    
Reference rate for variable interest rate         LIBOR               LIBOR        
Term of extension         1 year                        
Variable interest rate spread (as a percent)         2.00%     1.75% 0.75% 3.00% 1.10%   2.20% 1.95% 3.20%    
Line of credit, average interest rate (as a percent)         2.76% 2.96%                      
Outstanding borrowings under the line of credit         255,000 290,000                      
Maximum expanded revolving line of credit         2,000,000                        
Fair value of outstanding line of credit         $ 256,132 $ 292,366