Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details) - USD ($)
 3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Principal balance outstanding $ 2,615,211,000 $ 2,809,568,000
Total senior unsecured notes, net $ 2,630,614,000 2,826,110,000
4.500% Senior Unsecured Notes Due April 18, 2022 [Member]    
Debt Instrument [Line Items]    
Interest rate of senior unsecured notes 4.50%  
Maturity date of the senior unsecured notes Apr. 18, 2022  
3.150% Senior Unsecured Notes, Due May 15, 2023 [Member]    
Debt Instrument [Line Items]    
Interest rate of senior unsecured notes 3.15%  
Maturity date of the senior unsecured notes May 15, 2023  
Unsecured Note [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 575,000,000 575,000,000
Adjustment for unamortized debt discount (3,338,000) (3,505,000)
Unamortized deferred financing costs (2,224,000) (2,350,000)
Total senior unsecured notes, net 569,438,000 569,145,000
Unsecured Note [Member] | 4.500% Senior Unsecured Notes Due April 18, 2022 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 300,000,000 300,000,000
Effective rate [1] 4.612%  
Unsecured Note [Member] | 3.150% Senior Unsecured Notes, Due May 15, 2023 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 275,000,000 $ 275,000,000
Effective rate [1] 3.517%  
[1] Includes the cost of terminated treasury lock agreements (if any), offering and other transaction costs and the discount/premium on the notes, as applicable.