Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Principal balance outstanding $ 2,340,009,000 $ 2,145,393,000
Adjustment for unamortized debt discount (4,430,000)  
Total senior unsecured notes, net 2,340,009,000 2,154,920,000
5.800% Senior Unsecured Notes, Due January 15, 2016 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding $ 200,000,000  
Interest rate of senior unsecured notes 5.80%  
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  
7.750% Senior Unsecured Notes, Due August 15, 2019 [Member]    
Debt Instrument [Line Items]    
Interest rate of senior unsecured notes 7.75%  
Maturity date of the senior unsecured notes Aug. 15, 2019  
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 1,275,000,000
Adjustment for unamortized debt discount (4,430,000) (6,156,000)
Unamortized deferred financing costs (3,215,000) (5,062,000)
Total senior unsecured notes, net $ 817,355,000 1,263,782,000
Unsecured Note [Member] | 5.800% Senior Unsecured Notes, Due January 15, 2016 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding [1]   200,000,000
Effective rate [1],[2] 5.806%  
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 [2] 2.803%  
Unsecured Note [Member] | 7.750% Senior Unsecured Notes, Due August 15, 2019 [Member]    
Debt Instrument [Line Items]    
Principal balance outstanding [3]   250,000,000
Effective rate [2],[3] 8.017%  
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] On January 15, 2016, the Company repaid these notes at maturity using proceeds from a new unsecured term loan 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.
[3] During the year ended December 31, 2016, the Company purchased and redeemed these notes. See summaries above.