Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 0.7	$ 10.2
Calls/ Maturities/ Paydowns	(0.4)	(9.6)
Purchases	5.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.1
Net Transfers In (Out)	0.0	0.0
Ending Fair value	5.3	0.7
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(9.6)
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Ending Fair value	0.0	0.3
Fixed maturities \| Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(9.6)
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Ending Fair value	0.0	0.3
Equity securities \| Nonredeemable preferred stocks \| Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.0	0.0
Calls/ Maturities/ Paydowns	0.0	0.0
Purchases	5.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.0
Net Transfers In (Out)	0.0	0.0
Ending Fair value	5.0	0.0
Equity securities \| Common equities \| Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.4	0.3
Calls/ Maturities/ Paydowns	(0.1)	0.0
Purchases	0.0	0.0
Sales	0.0	0.0
Net Realized (Gain)/Loss on Sales	0.0	0.0
Change in Valuation	0.0	0.1
Net Transfers In (Out)	0.0	0.0
Ending Fair value	$ 0.3	$ 0.4

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

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- Definition

Amount of gain (loss) recognized in the income statement for 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income (loss) for 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- 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

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of settlement 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

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

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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

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-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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