Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Unrealized gains and losses on securities:
Net realized investment gains (losses)	$ 40.4	$ (24.7)	$ 74.9	$ (37.8)
Income tax provision	12.1	8.9	25.1	10.3
Foreign currency exchange gains (losses)	10.3	(4.4)	7.4	(5.7)
Total, net of taxes	16.2	(69.8)	17.2	(99.6)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Total, net of taxes	4.5	(13.7)	27.4	(16.9)
Unrealized Gains and Losses on Securities | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized investment gains (losses)	0.0	(15.2)	(5.4)	(19.2)
Income tax provision	0.0	1.5	1.0	2.3
Foreign Currency Translation Adjustments | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Foreign currency exchange gains (losses)	$ 4.5	$ 0.0	$ 31.8	$ 0.0