Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Unrealized gains and losses on securities:
Net of taxes	$ (5.3)	$ 2.6
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment (gains) loss	(3.4)	3.1
Benefit for income taxes	(1.9)	(0.5)
Net of taxes	$ (5.3)	$ 2.6

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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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