Retirement Benefits - Schedule of Changes in Fair Value of Level 3 Investments (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($)
Change in fair value of assets:
Fair value - beginning of period	$ 931.4
Fair value - end of period	925.1
Significant Unobservable Inputs (Level 3)
Change in fair value of assets:
Fair value - beginning of period	1.2
Unrealized Gains (Losses) on Plan Assets	0.1
Purchases, Issuances, and Settlements	8.2
Fair value - end of period	9.5
Significant Unobservable Inputs (Level 3) \| Real estate investments
Change in fair value of assets:
Fair value - beginning of period	0.7
Unrealized Gains (Losses) on Plan Assets	0.0
Purchases, Issuances, and Settlements	(0.2)
Fair value - end of period	0.5
Significant Unobservable Inputs (Level 3) \| Cash and cash equivalents \| Domestic equity securities
Change in fair value of assets:
Fair value - beginning of period	0.5
Unrealized Gains (Losses) on Plan Assets	0.1
Purchases, Issuances, and Settlements	1.6
Fair value - end of period	2.2
Significant Unobservable Inputs (Level 3) \| Fixed income: \| Corporate debt securities
Change in fair value of assets:
Fair value - beginning of period	0.0
Unrealized Gains (Losses) on Plan Assets	0.0
Purchases, Issuances, and Settlements	6.8
Fair value - end of period	$ 6.8

Defined Benefit Plan Fair Value Of Plan Assets Based On Unobservable Inputs Unrealized Gains Losses

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930

Reference 2: 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

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6749337

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Plan Assets
-URI http://asc.fasb.org/extlink&oid=6520858

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- Definition

Amount of increase (decrease) in plan assets from purchases, sales and settlement of plan obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920

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