Investments - Allowance for Credit Losses (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($)
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	$ 0.0
Securities for which allowance was not previously recorded	15.9
Securities sold during the period	(39.3)
Additional net increases (decreases) in existing allowance	23.1
Ending balance, December 31, 2020	6.6
Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.3
Securities sold during the period	(0.2)
Additional net increases (decreases) in existing allowance	0.1
Ending balance, December 31, 2020	0.2
Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.3
Securities sold during the period	0.0
Additional net increases (decreases) in existing allowance	(0.2)
Ending balance, December 31, 2020	0.1
Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	15.3
Securities sold during the period	(39.0)
Additional net increases (decreases) in existing allowance	23.0
Ending balance, December 31, 2020	6.1
Asset backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Securities for which allowance was not previously recorded	0.0
Securities sold during the period	(0.1)
Additional net increases (decreases) in existing allowance	0.2
Ending balance, December 31, 2020	0.2
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	6.9
Cumulative Effect, Period of Adoption, Adjustment \| Foreign Governments
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Cumulative Effect, Period of Adoption, Adjustment \| Obligations of states and political subdivisions
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	0.0
Cumulative Effect, Period of Adoption, Adjustment \| Corporate bonds
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	6.8
Cumulative Effect, Period of Adoption, Adjustment \| Asset backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Beginning balance, January 1, 2020	$ 0.1

Debt Securities, Available-for-sale, Allowance for Credit Loss, Additional Period Increase (Decrease)

No definition available.

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- Definition

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=121558606&loc=SL82898722-210454

+ Details

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- Definition

Amount of expense for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), for which no credit loss was previously recorded.

+ References

+ Details

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- Definition

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

+ References

No definition available.

+ Details

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- Definition

Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from sale.

+ References

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