Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified from Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Unrealized gains and losses on securities:
Net realized investment gains (losses)	$ (17.9)	$ (34.5)
Provision for income taxes	(8.5)	13.0
Net realized foreign currency transaction losses	(2.7)	(2.9)
Net income (loss)	(33.8)	(1.0)
Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net income (loss)	(17.5)	(22.9)
Unrealized Gains and Losses on Securities | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized investment gains (losses)	(22.2)	5.4
Provision for income taxes	4.7	(1.0)
Foreign Currency Translation Adjustments | Reclassification out of Accumulated Other Comprehensive Income
Unrealized gains and losses on securities:
Net realized foreign currency transaction losses	$ 0.0	$ (27.3)