Quarterly report pursuant to Section 13 or 15(d)

Senior Unsecured Notes (Details)

v2.4.0.6
Senior Unsecured Notes (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Long-Term Debt [Line Items]    
Senior Unsecured Notes $ 1,446,987 $ 1,446,894
4.600% Senior Unsecured Notes, Due June 15, 2013 [Member]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 99,994 99,987
Effective Rate 4.742% [1]  
Interest rate of senior unsecured notes 4.60%  
Maturity date of the Senior Unsecured Notes Jun. 15, 2013  
5.125% Senior Unsecured Notes, Due February 15, 2014 [Member]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 200,210 200,270
Effective Rate 5.11% [1]  
Interest rate of senior unsecured notes 5.125%  
Maturity date of the Senior Unsecured Notes Feb. 15, 2014  
5.125% Senior Unsecured Notes, Due January 15, 2015 [Member]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 149,833 149,810
Effective Rate 5.297% [1]  
Interest rate of senior unsecured notes 5.125%  
Maturity date of the Senior Unsecured Notes Jan. 15, 2015  
5.800% Senior Unsecured Notes, Due January 15, 2016 [Member]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 200,218 200,237
Effective Rate 5.806% [1]  
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]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 248,633 248,560
Effective Rate 2.803% [1]  
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]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes 248,639 248,585
Effective Rate 8.017% [1]  
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]
    
Long-Term Debt [Line Items]    
Senior Unsecured Notes $ 299,460 $ 299,445
Effective Rate 4.612% [1]  
Interest rate of senior unsecured notes 4.50%  
Maturity date of the Senior Unsecured Notes Apr. 18, 2022  
[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.