Retirement Benefits - Narrative - Plan Assets and Investment Policy (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Private assets
Defined Benefit Plan Disclosure [Line Items]
Increase in plan assets	$ 0.3	$ 11.3
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Increase in plan assets		$ 0.3

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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