Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unrealized gains and losses on securities:
Net of taxes	$ 8.7	$ 4.9
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment loss (gains)	9.9	5.4	$ (41.6)
(Benefit) provision for income taxes	(1.2)	(0.5)	13.4
Net of taxes	$ 8.7	$ 4.9	$ (28.2)

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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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