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Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) - USD ($) $ in Millions | 12 Months Ended
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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

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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 + Details
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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 + Details
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