Senior Unsecured Notes (Summary Of Senior Unsecured Notes) (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Dec. 31, 2010
Senior unsecured notes$ 1,119,063$ 1,118,451
5.250% Senior Unsecured Notes, Due January 15, 2012 [Member]
  
Senior unsecured notes99,93999,793
Effective Rate5.457%[1] 
Interest rate of the senior unsecured notes5.25% 
Maturity date of the senior unsecured notesJan. 15, 2012 
6.150% Senior Unsecured Notes, Due December 15, 2012 [Member]
  
Senior unsecured notes94,31593,946
Effective Rate6.894%[1] 
Interest rate of the senior unsecured notes6.15% 
Maturity date of the senior unsecured notesDec. 15, 2012 
5.820% Senior Unsecured Notes, Due March 15, 2013 [Member]
  
Senior unsecured notes25,94425,861
Effective Rate6.448%[1] 
Interest rate of the senior unsecured notes5.82% 
Maturity date of the senior unsecured notesMar. 15, 2013 
4.600% Senior Unsecured Notes, Due June 15, 2013 [Member]
  
Senior unsecured notes99,95199,930
Effective Rate4.742%[1] 
Interest rate of the senior unsecured notes4.60% 
Maturity date of the senior unsecured notesJun. 15, 2013 
5.125% Senior Unsecured Notes, Due February 15, 2014 [Member]
  
Senior unsecured notes200,570200,749
Effective Rate5.11%[1] 
Interest rate of the senior unsecured notes5.125% 
Maturity date of the senior unsecured notesFeb. 15, 2014 
5.125% Senior Unsecured Notes, Due January 15, 2015 [Member]
  
Senior unsecured notes149,694149,625
Effective Rate5.297%[1] 
Interest rate of the senior unsecured notes5.125% 
Maturity date of the senior unsecured notesJan. 15, 2015 
5.800% Senior Unsecured Notes, Due January 15, 2016 [Member]
  
Senior unsecured notes200,332200,389
Effective Rate5.806%[1] 
Interest rate of the senior unsecured notes5.80% 
Maturity date of the senior unsecured notesJan. 15, 2016 
7.750% Senior Unsecured Notes, Due August 15, 2019 [Member]
  
Senior unsecured notes$ 248,318$ 248,158
Effective Rate8.017%[1] 
Interest rate of the senior unsecured notes7.75% 
Maturity date of the senior unsecured notesAug. 15, 2019 
[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.