Derivative Financial Instruments - Contractual and Weighted-Average Interest Rates, Which Include the Effects of Hedges Related to Financial Instruments (Details) - Long-term debt	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Contractual rates	3.34%	3.40%
Rate including impact of hedges	2.66%	2.29%

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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