Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Unrealized gains and losses on securities:
Net of taxes			$ (19.2)	$ 4.2
Unrealized Gains and Losses on Securities
Unrealized gains and losses on securities:
Net realized investment (gains) loss	$ (16.5)	$ 1.8	(19.9)	4.9
Provision (benefit) for income taxes	2.6	(0.2)	0.7	(0.7)
Net of taxes	$ (13.9)	$ 1.6	$ (19.2)	$ 4.2

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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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