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Long-Term Debt (Narrative 2013 Senior Notes) (Details) - Senior Notes - 2013 Senior Notes - USD ($)
Mar. 31, 2018
Dec. 17, 2013
Debt Instrument [Line Items]    
Debt instrument, face amount   $ 400,000,000.0
Debt instrument, stated interest rate (as percent) 5.375% 5.375%