Segment Information (Identifiable Assets by Business Segment) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Identifiable assets	$ 15,334.2	$ 15,469.6
Discontinued operations	89.3	88.0
U.S. Companies [Member]
Segment Reporting Information [Line Items]
Identifiable assets	10,820.0	10,909.2
Chaucer [Member]
Segment Reporting Information [Line Items]
Identifiable assets	$ 4,424.9	$ 4,472.4

Amount classified as assets attributable to disposal group held for sale or disposed of.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2941-110230

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=65878427&loc=d3e1107-107759

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65878427&loc=SL51721533-107759

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