Debt and Credit Arrangements - Fair Value Disclosures about Debt (Details) - Convertible Debt	Jun. 30, 2018	Dec. 31, 2017
Convertible Notes, due 2024
Debt Instrument
Debt instrument percentage over par value	122.00%	105.00%
Convertible Notes, Due 2018
Debt Instrument
Debt instrument percentage over par value	101.00%	99.00%

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