Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Unrealized gains and losses on securities:
Net investment and other (gains) losses	$ 115.9	$ (72.4)	$ (26.0)
Income tax provision (benefit)	(8.0)	(1.4)	7.7
Foreign currency exchange gains (losses)	5.0	(1.6)	(15.4)
Total, net of taxes	175.2	(6.7)	54.1
Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Total, net of taxes	66.2	(8.0)	(12.8)
Unrealized Holding Gains (Losses) on Securities | Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Net investment and other (gains) losses	43.6	(12.2)	(12.8)
Income tax provision (benefit)	(9.2)	4.2	0.0
Foreign Currency Translation Adjustments | Reclassification out of accumulated other comprehensive income
Unrealized gains and losses on securities:
Foreign currency exchange gains (losses)	$ 31.8	$ 0.0	$ 0.0