Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Unrealized gains and losses on securities:
Net of taxes			$ (15.2)	$ 6.7
Unrealized Gains and Losses on Securities
Unrealized gains and losses on securities:
Net realized investment (gains) loss	$ 4.5	$ 2.8	(15.4)	7.7
Provision (benefit) for income taxes	(0.5)	(0.3)	0.2	(1.0)
Net of taxes	$ 4.0	$ 2.5	$ (15.2)	$ 6.7

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Amount before tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

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Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income (loss).

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