Fair Value Measurements - Narrative (Details) - Recurring Basis [Member] - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-term Debt, Fair Value	$ 2,202.0	$ 2,262.0
Other Assets [Member] | Egypt, Pounds
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Assets	1.7	2.2
Other Noncurrent Assets [Member] | Angola, Kwanza
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other Assets	$ 2.9	$ 4.3