Retirement Benefits - Narrative - Plan Assets and Investment Policy (Details) - USD ($) $ in Millions	12 Months Ended	13 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Private assets
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Fair Value of Plan Assets, Period Increase (Decrease)	$ 11.3	$ (8.6)

The amount of increase or decrease in the fair value of plan assets during the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssetsPeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration