Annual report pursuant to Section 13 and 15(d)

Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details)

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Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Principal balance outstanding $ 2,809,568,000 $ 2,340,009,000
Adjustment for unamortized debt discount (3,505,000)  
Total senior unsecured notes, net $ 2,809,568,000  
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 $ 575,000,000 825,000,000
Adjustment for unamortized debt discount (3,505,000) (4,430,000)
Unamortized deferred financing costs (2,350,000) (3,215,000)
Total senior unsecured notes, net $ 569,145,000 817,355,000
Unsecured Note [Member] | 2.500% Senior Unsecured Notes Due December 15, 2017 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding [1]   250,000,000
Effective rate [1],[2] 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 [2] 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 [2] 3.517%  
[1] The Company repaid these notes at maturity using available cash and borrowings under the Company's unsecured revolving credit facility.
[2] Includes the cost of terminated treasury lock agreements (if any), offering and other transaction costs and the discount/premium on the notes, as applicable.