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Debt (Narrative) (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]  
Credit facility, borrowing base $ 249.6
Letters of Credit Outstanding, Amount 52.3
Unsecured Senior Notes Due 2024 [Member]  
Debt Instrument [Line Items]  
Senior Notes $ 500.0
Stated interest rate on unsecured senior notes 7.75%
Unsecured Senior Notes Due 2021 [Member]  
Debt Instrument [Line Items]  
Senior Notes $ 800.0
Stated interest rate on unsecured senior notes 7.125%