Employee Benefit Plans and Other Postretirement Benefits - Summary of Changes in Pension Plans' Level 3 Assets (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	$ 830.4
Fair value of plan assets at plan year end	954.8
Significant unobservable input (Level 3)
Level 3 fair value of pension plans' assets [Roll Forward]
Plan assets at fair value at beginning of period	5.7
Sales	(1.5)
Fair value of plan assets at plan year end	$ 4.2

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.

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=82843861&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant