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Debt And Interest Expense Debt and Interest Expense (Senior Notes) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Aug. 15, 2007
Debt Instrument [Line Items]        
Debt Instrument, Maturity Date Apr. 01, 2020      
Debt Instrument, Interest Rate, Stated Percentage       3.25%
Debt Instrument, Face Amount   $ 1,250,000 $ 1,250,000 $ 1,250,000
issuance cost on senior note   (17,471) (12,811)  
Senior Notes   $ 1,782,529 1,237,189  
Due 2023 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Offering Date   Apr. 16, 2013    
Debt Instrument, Maturity Date   May 01, 2023    
Debt Instrument, Interest Rate, Stated Percentage   4.625%    
Debt Instrument, Face Amount   $ 750,000 750,000  
Due 2025 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Offering Date   Mar. 27, 2015    
Debt Instrument, Maturity Date   Apr. 01, 2025    
Debt Instrument, Interest Rate, Stated Percentage   5.25%    
Debt Instrument, Face Amount   $ 500,000 500,000  
Due 2027 [Member]        
Debt Instrument [Line Items]        
Debt Instrument, Offering Date   Jul. 05, 2017    
Debt Instrument, Maturity Date   Jul. 15, 2027    
Debt Instrument, Interest Rate, Stated Percentage   4.75%    
Debt Instrument, Face Amount   $ 550,000 $ 0