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Fair Value of Financial Instruments - Narrative (Details) - Senior Notes Due 2021 - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Debt instruments at fair value $ 297.3 $ 282.2
Interest rate, stated percentage (as a percent) 12.75%