Accumulated Other Comprehensive (Loss) Income - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Unrealized gains and losses on securities:
Net of taxes			$ 4.2	$ 2.4
Unrealized Gains and Losses on Securities [Member]
Unrealized gains and losses on securities:
Net realized investment loss	$ 1.8	$ 15.1	4.9		$ 2.6
Benefit for income taxes	(0.2)	(2.5)	(0.7)		(0.2)
Net of taxes	$ 1.6	$ 12.6	$ 4.2	$ 2.4	$ 2.4

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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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