Fair Value Disclosures - Summary of the Change in Balance Sheet Carrying Value Associated with Level 3 Financial Assets Carried at Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	$ 12.7	$ 18.8
Total gains (losses) (realized/ unrealized) included in earnings	(0.1)	(0.1)
Net unrealized (losses) gains included in other comprehensive income	0.6	(0.5)
Purchases	8.5	24.6
Sales	(0.5)	(4.4)
Transfers in	1.0	0.1
Transfers out	(9.4)	(25.8)
Balance, end of period	12.8	12.7
Other assets
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	0.1	0.2
Total gains (losses) (realized/ unrealized) included in earnings	(0.1)	(0.1)
Net unrealized (losses) gains included in other comprehensive income	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	0.0	0.1
Asset-backed
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	0.0	4.7
Total gains (losses) (realized/ unrealized) included in earnings	0.0	0.0
Net unrealized (losses) gains included in other comprehensive income	0.0	0.0
Purchases	8.5	23.9
Sales	0.0	(3.0)
Transfers in	0.0	0.0
Transfers out	(8.5)	(25.6)
Balance, end of period	0.0	0.0
Commercial mortgage-backed
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	3.3	3.3
Total gains (losses) (realized/ unrealized) included in earnings	0.0	0.1
Net unrealized (losses) gains included in other comprehensive income	0.2	(0.1)
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	3.5	3.3
U.S. corporate
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	0.0	0.2
Total gains (losses) (realized/ unrealized) included in earnings	0.0	0.0
Net unrealized (losses) gains included in other comprehensive income	0.0	0.0
Purchases	0.0	0.7
Sales	0.0	(0.8)
Transfers in	1.0	0.1
Transfers out	(0.9)	(0.2)
Balance, end of period	0.1	0.0
Foreign corporate
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	8.2	9.3
Total gains (losses) (realized/ unrealized) included in earnings	0.0	(0.1)
Net unrealized (losses) gains included in other comprehensive income	0.4	(0.4)
Purchases	0.0	0.0
Sales	(0.5)	(0.6)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	8.1	8.2
Non-redeemable preferred stocks
Changes in balance sheet carrying value associated with Level 3 financial assets carried at fair value
Balance, beginning of period	1.1	1.1
Total gains (losses) (realized/ unrealized) included in earnings	0.0	0.0
Net unrealized (losses) gains included in other comprehensive income	0.0	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Balance, end of period	$ 1.1	$ 1.1

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

No definition available.

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationCalculationRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_OtherAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DomesticCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ForeignCorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_NonredeemablePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: