Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Unrealized gains and losses on securities:
Net realized investment (gains) losses	$ (72.4)	$ (26.0)	$ (120.9)
Income tax provision (benefit)	(1.4)	7.7	14.1
Net of taxes	(6.7)	54.1	14.1
Unrealized Holding Gains on Securities | Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Net realized investment (gains) losses	(12.2)	(12.8)	9.9
Income tax provision (benefit)	4.2	0.0	(1.2)
Net of taxes	$ (8.0)	$ (12.8)	$ 8.7