Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details) - USD ($)
 6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Principal balance outstanding $ 2,950,219,000 $ 2,340,009,000
Total senior unsecured notes, net $ 2,968,699,000 2,357,055,000
5.800% Senior Unsecured Notes, Due January 15, 2016 [Member]    
Debt Instrument [Line Items]    
Maturity date of the senior unsecured notes Jan. 15, 2016  
2.500% Senior Unsecured Notes Due December 15, 2017 [Member]    
Debt Instrument [Line Items]    
Interest rate of senior unsecured notes 2.50%  
Maturity date of the senior unsecured notes Dec. 15, 2017  
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 $ 825,000,000 825,000,000
Adjustment for unamortized debt discount (3,949,000) (4,430,000)
Unamortized deferred financing costs (2,757,000) (3,215,000)
Total senior unsecured notes, net 818,294,000 817,355,000
Unsecured Note [Member] | 2.500% Senior Unsecured Notes Due December 15, 2017 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 250,000,000 250,000,000
Effective rate [1] 2.803%  
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.