Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 56.8	$ 56.2
Transfers into Level 3		5.6
Transfers out of Level 3	(10.0)	(13.2)
Included in net income	0.9	(0.5)
Included in other comprehensive income	(0.4)	0.8
Purchases	1.6	10.6
Sales		(1.6)
Settlements	(6.2)	(1.1)
Ending balance	42.7	56.8
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	(0.8)
Credit Financial
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	50.4	40.7
Transfers into Level 3		5.6
Transfers out of Level 3	(10.0)	(5.6)
Included in net income	0.8	(0.1)
Included in other comprehensive income	(0.4)	0.8
Purchases	0.4	10.6
Sales		(0.5)
Settlements	(6.2)	(1.1)
Ending balance	35.0	50.4
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	0.0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	6.4	15.5
Transfers into Level 3		0.0
Transfers out of Level 3	0.0	(7.6)
Included in net income	0.1	(0.4)
Included in other comprehensive income	0.0	0.0
Purchases	0.1	0.0
Sales		(1.1)
Settlements	0.0	0.0
Ending balance	6.6	6.4
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0	(0.8)
Other Invested Assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers out of Level 3	0.0
Included in net income	0.0
Included in other comprehensive income	0.0
Purchases	1.1
Settlements	0.0
Ending balance	1.1	$ 0.0
Amount of total gains or losses for the year included in net income attributable to the change in unrealized gains or losses relating to assets still held at end of period	$ 0.0