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Senior Notes (Summary of Senior Notes) (Parenthetical) (Details) - USD ($)
$ in Thousands
1 Months Ended 9 Months Ended
Oct. 27, 2017
Jul. 31, 2016
Sep. 30, 2019
Dec. 31, 2018
Oct. 31, 2017
Dec. 31, 2015
Jul. 31, 2014
Debt Instrument [Line Items]              
Senior notes     $ 1,016,751 $ 1,015,973      
Senior notes 4.250% due 2024 [Member]              
Debt Instrument [Line Items]              
Stated interest rate     4.25%        
Senior notes             $ 300,000
Debt instrument, maturity date     Jul. 31, 2024        
Redemption price, percentage of principal amount     100.00%        
Additional issuance of long-term debt   $ 200,000          
Senior notes 3.50% due 2020 [Member]              
Debt Instrument [Line Items]              
Stated interest rate     3.50%        
Senior notes           $ 300,000  
Debt instrument, maturity date     Dec. 31, 2020        
Redemption price, percentage of principal amount     100.00%        
Senior notes 5.20% due 2047 [Member]              
Debt Instrument [Line Items]              
Stated interest rate     5.20%        
Senior notes         $ 200,000    
Debt instrument, maturity date     Oct. 31, 2047        
Redemption price, percentage of principal amount     100.00%        
Additional issuance of long-term debt $ 25,000