Goodwill - Changes to Carrying Value by Segment (Details) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($)
Changes in carrying amounts of goodwill
December 31, 2016	$ 431.4
Foreign currency translation	0.3
September 30, 2017	431.7
Region One
Changes in carrying amounts of goodwill
December 31, 2016	368.7
Foreign currency translation	0.3
September 30, 2017	369.0
Region Two
Changes in carrying amounts of goodwill
December 31, 2016	62.7
Foreign currency translation	0.0
September 30, 2017	$ 62.7

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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