Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 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	0.7	5.6
Transfers out of Level 3	$ (10.1)	(13.2)
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Gain (Loss), Statement of Income or Comprehensive Income [Extensible Enumeration]	Net realized investment and other gains (losses)
Fair Value Recurring Basis Unobservable Input Reconciliation Asset Gain Loss Statement Of Other Comprehensive Income Extensible List Not Disclosed Flag	Included in other comprehensive income
Total gains or losses (realized/unrealized):
Included in net income	$ 29.8	(0.5)
Included in other comprehensive income	(0.7)	0.8
Purchases, issuances, sales, and settlements:
Purchases	91.7	10.6
Sales	(1.7)	(1.6)
Settlements	(9.9)	(1.1)
Ending balance	156.6	56.8
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	28.4	(0.8)
Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	50.4
Transfers into Level 3	0.7
Transfers out of Level 3	(10.1)
Total gains or losses (realized/unrealized):
Included in net income	1.5
Included in other comprehensive income	(0.7)
Purchases, issuances, sales, and settlements:
Purchases	2.5
Sales	0.0
Settlements	(7.7)
Ending balance	36.6	50.4
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	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	0.0
Transfers out of Level 3	0.0	(7.6)
Total gains or losses (realized/unrealized):
Included in net income	28.3	(0.4)
Included in other comprehensive income	0.0	0.0
Purchases, issuances, sales, and settlements:
Purchases	80.9	0.0
Sales	(0.1)	(1.1)
Settlements	0.0	0.0
Ending balance	115.5	6.4
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	28.4	(0.8)
Other Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers into Level 3	0.0
Transfers out of Level 3	0.0
Total gains or losses (realized/unrealized):
Included in net income	0.0
Included in other comprehensive income	0.0
Purchases, issuances, sales, and settlements:
Purchases	8.3
Sales	(1.6)
Settlements	(2.2)
Ending balance	4.5	0.0
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period	0.0
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		(5.6)
Total gains or losses (realized/unrealized):
Included in net income		(0.1)
Included in other comprehensive income		0.8
Purchases, issuances, sales, and settlements:
Purchases		10.6
Sales		(0.5)
Settlements		(1.1)
Ending balance		50.4
Amount of total gains or losses for the year included in net income (loss) attributable to the change in unrealized gains or losses relating to assets still held at end of period		$ 0.0