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Fair Value Measurements (Additional Information Regarding Fair Value Measurements) (Details) - USD ($)
Sep. 30, 2019
Nov. 15, 2017
Senior Notes Due 2019 [Member] | Level 2 [Member] | Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Closing trading price per $100 as of the last day of trading for the quarter $ 100  
Unsecured Debt [Member] | 2027 Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Debt instrument, face amount 750,000,000.0 $ 750,000,000.0
Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Carrying Value 742,704,000  
Senior Notes [Member] | Level 2 [Member] | Fair Value [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Debt, Fair Value $ 811,485,000