Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Unrealized gains and losses on securities:
Net realized investment gains (losses)	$ 44.7	$ 5.3	$ 119.6	$ (32.5)
Income tax provision (benefit)	(4.0)	5.8	21.1	16.1
Foreign currency exchange gains (losses)	9.1	1.3	16.5	(4.4)
Net income (loss)	(48.8)	22.4	(66.0)	122.0
Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net income (loss)	37.7	7.8	65.1	(9.1)
Unrealized Gains and Losses on Securities | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized investment gains (losses)	47.5	8.2	42.1	(11.0)
Income tax provision (benefit)	(9.8)	(0.4)	(8.8)	1.9
Foreign Currency Translation Adjustments | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Foreign currency exchange gains (losses)	$ 0.0	$ 0.0	$ 31.8	$ 0.0