Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Unrealized gains and losses on securities:
Net realized investment gains	$ (34.5)	$ 13.1
Income tax provision	13.0	1.4
Net realized investment and other gains (losses)	(40.2)	(1.3)
Net income (loss)	(1.0)	29.8
Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net income (loss)	(22.9)	3.2
Unrealized Gains and Losses on Securities | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized investment gains	5.4	4.0
Income tax provision	1.0	0.8
Foreign Currency Translation Adjustments | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized investment and other gains (losses)	$ 27.3	$ 0.0