Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Unrealized gains and losses on securities:
Net of taxes			$ 6.7	$ 0.3
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment loss (gains)	$ 2.8	$ (2.4)	7.7		$ 0.2
(Benefit) provision for income taxes	(0.3)	0.3	(1.0)		0.1
Net of taxes	$ 2.5	$ (2.1)	$ 6.7	$ 0.3	$ 0.3

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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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