Allowance For Credit Losses - Changes in CECL Allowance for Reinsurance Recoverables (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Jan. 01, 2020
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0.0
Cumulative effect of adoption	1.6	$ 3,260.1
Change in allowance	(0.1)
Ending balance	1.6
Impact of adoption
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]
Beginning balance	1.7
Cumulative effect of adoption	$ 1.7	$ (1.7)

Amount of allowance for credit loss on reinsurance recoverable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=121599337&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121599878&loc=SL82895884-210446

Name:	us-gaap_ReinsuranceRecoverablesAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant