v2.4.0.6
Stock-Based Compensation (Details) (USD $)
 12 Months Ended
Dec. 31, 2011
plan
Dec. 31, 2010
Dec. 31, 2009
Stock-Based Compensation      
Number of share-based compensation plans covering employees, officers and directors 4    
Number of share-based compensation plans for executive officers 1    
Number of share-based compensation plans for directors 2    
Employee Benefit Plan      
Maximum 401(K) plan contribution by employer as a percent of employee's salary 3.00%    
Aggregate employer 401(K) plan contribution during the period $ 267,000 $ 200,000 $ 189,000
Executive Deferred Stock Compensation Plan
      
Stock-Based Compensation      
Number of shares reserved for issuance 500,000    
Number of shares available for future issuance 500,000    
Nonemployee Directors' Deferred Stock Compensation Plan
      
Stock-Based Compensation      
Number of shares reserved for issuance 500,000    
Number of shares available for future issuance 443,135    
Stock Options
      
Stock-Based Compensation      
Vesting period, minimum (in years) P2Y    
Vesting period, maximum (in years) P3Y    
Compensation cost 4,200,000 3,100,000 2,900,000
Unrecognized compensation cost related to nonvested awards 1,300,000    
Unrecognized compensation cost related to nonvested awards assumed in the NHP acquisition 100,000    
Weighted average period over which cost is recognized (in years) 1    
Proceeds from options exercised 2,500,000 10,900,000 2,200,000
Fair value assumptions of stock options using Black-Scholes option pricing model      
Risk-free interest rate, maximum (as a percent) 2.78% 3.45% 2.32%
Risk-free interest rate, minimum (as a percent) 1.22% 2.00% 1.37%
Dividend yield (as a percent) 6.75% 6.75% 5.75%
Volatility factors of the expected market price for our common stock, maximum (as a percent) 44.30% 44.60% 42.70%
Volatility factors of the expected market price for our common stock, minimum (as a percent) 35.70% 37.10% 36.10%
Weighted average expected life of options, maximum (in years) 7.0 7.0 6.0
Weighted average expected life of options, minimum (in years) 4.25 4.25 3.5
Expiration period of stock options (in years) 10Y    
Stock Option, Shares      
Outstanding at the beginning of the period (in shares) 1,656,558    
Granted (in shares) 376,451    
Options assumed from NHP (in shares) 108,785    
Options exercised (in shares) (94,789)    
Outstanding at the end of the period (in shares) 2,047,005 1,656,558  
Exercisable at the end of the period (in shares) 1,685,965    
Stock Option, Weighted Average Exercise Price      
Outstanding at the beginning of the period (in dollars per share) $ 38.12    
Options granted (in shares) $ 53.64    
Options assumed from NHP (in dollars per share) $ 48.60    
Options exercised (in dollars per share) $ 26.00    
Outstanding at the end of the period (in dollars per share) $ 42.10 $ 38.12  
Exercisable at the end of the period (in dollars per share) $ 40.22    
Stock Option, Weighted Average Remaining Contractual Life (years)      
Outstanding at the end of the period (in years) 6.9    
Exercisable at the end of the period (in years) 6.5    
Stock Option, Intrinsic Value      
Outstanding at the end of the period 26,734,000    
Exercisable at the end of the period 25,158,000    
Nonvested Stock Options, Shares      
Nonvested stock options at the beginning of the period (in shares) 340,203    
Granted (in shares) 376,451    
Assumed from NHP (in shares) 108,785    
Vested (in shares) (464,399)    
Nonvested stock options at the end of the period (in shares) 361,040 340,203  
Nonvested Stock Options, Weighted Average Grant Date Fair Value      
Nonvested stock options at the beginning of the period (in dollars per share) $ 8.33    
Granted (in dollars per share) $ 11.17    
Assumed from NHP (in dollars per share) $ 9.93    
Vested (in dollars per share) $ 9.12    
Nonvested stock options at the end of the period (in dollars per share) $ 10.76 $ 8.33  
Stock Options | Maximum
      
Stock Option, Range of Exercise Prices      
Range of exercise price, high end of range (in dollars per share) $ 57.19 $ 45.26  
Exercise price range, options granted (in dollars per share) $ 57.19    
Exercise price range, options assumed from NHP (in dollars per share) $ 48.60    
Exercise price range, options excercised (in dollars per share) $ 48.60    
Exercise price range, options excercisable (in dollars per share) $ 57.19    
Stock Options | Minimum
      
Stock Option, Range of Exercise Prices      
Range of exercise price, low end of range (in dollars per share) $ 11.45 $ 11.34  
Exercise price range, options granted (in dollars per share) $ 52.48    
Exercise price range, options assumed from NHP (in dollars per share) $ 48.60    
Exercise price range, options excercised (in dollars per share) $ 11.34    
Exercise price range, options excercisable (in dollars per share) $ 11.45    
Restricted Stock and Restricted Stock Units
      
Stock-Based Compensation      
Vesting period, minimum (in years) P2Y    
Vesting period, maximum (in years) P5Y    
Unrecognized compensation cost related to nonvested awards 19,500,000    
Unrecognized compensation cost related to nonvested awards assumed in the NHP acquisition 300,000    
Weighted average period over which cost is recognized (in years) 3.1    
Restricted Stock and Restricted Stock Units | General and administrative expenses
      
Stock-Based Compensation      
Compensation cost $ 15,100,000 $ 11,000,000 $ 9,000,000
Restricted Stock
      
Nonvested Restricted Stock and Restricted Stock Units, Shares      
Nonvested at the beginning of the period (in shares) 493,967    
Granted (in shares) 393,764    
Assumed from NHP (in shares) 1,337    
Vested (in shares) (281,090)    
Forfeited (in shares) (15,780)    
Nonvested at the end of the period (in shares) 592,198    
Nonvested Restricted Stock and Restricted Stock Units, Weighted Average Grant Date Fair Value      
Nonvested at the beginning of the period (in dollars per share) $ 43.10    
Granted (in dollars per share) $ 53.33    
Assumed from NHP (in dollars per share) $ 53.74    
Vested (in dollars per share) $ 42.29    
Forfeited (in dollars per share) $ 51.22    
Nonvested at the end of the period (in dollars per share) $ 50.09    
Restricted Stock Units
      
Nonvested Restricted Stock and Restricted Stock Units, Shares      
Nonvested at the beginning of the period (in shares) 4,690    
Granted (in shares) 2,050    
Assumed from NHP (in shares) 41,495    
Vested (in shares) (14,946)    
Nonvested at the end of the period (in shares) 33,289    
Nonvested Restricted Stock and Restricted Stock Units, Weighted Average Grant Date Fair Value      
Nonvested at the beginning of the period (in dollars per share) $ 39.28    
Granted (in dollars per share) $ 52.48    
Assumed from NHP (in dollars per share) $ 53.74    
Vested (in dollars per share) $ 50.08    
Nonvested at the end of the period (in dollars per share) $ 53.27    
Employee and Director Stock Purchase Plan
      
Stock-Based Compensation      
Number of shares reserved for issuance 2,500,000    
Number of shares available for future issuance 2,455,762    
Number of shares purchased 44,238    
Employee and Director Stock Purchase Plan | Minimum
      
Stock-Based Compensation      
Purchase price of common stock shares with respect to the employee tax-favored portion of the plan (as a percent) 90.00%    
Purchase price of common stock shares with respect to the additional employee and director portion of the plan (as a percent) 95.00%    
2006 Incentive Plan
      
Stock-Based Compensation      
Number of shares reserved for issuance 5,000,000    
Number of shares available for future issuance 1,905,338    
2006 Stock Plan for Directors
      
Stock-Based Compensation      
Number of shares reserved for issuance 400,000    
Number of shares available for future issuance 165,313