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Financing Financing - Narrative (Details) - Line of Credit [Member] - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Commercial Paper [Member] | CP Program [Member]    
Short-term Debt [Line Items]    
Borrowings outstanding $ 55,500,000 $ 0
Maximum borrowing capacity 550,000,000 0
Revolving Credit Facility [Member] | 2017 Facility [Member]    
Short-term Debt [Line Items]    
Maximum borrowing capacity $ 550,000,000 $ 0