Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ 17.5
Beginning balance	$ 16.1	24.5
Transfers into Level 3	37.6	1.0
Transfers out of Level 3	$ 0.0	0.0
Fair Value Recurring Basis Unobservable Input Reconciliation Asset Gain Loss Statement Of Income Extensible List Not Disclosed Flag	Included in net income
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	$ (1.1)	4.2
Included in other comprehensive income	(4.8)	(0.8)
Purchases, issuances, sales, and settlements:
Purchases	10.0	1.3
Issuances	0.0	0.0
Sales	(3.0)	(14.1)
Settlements	0.0	0.0
Ending balance	56.2	16.1
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	(4.4)	0.0
Credit Financial
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		2.8
Beginning balance	2.8	7.0
Transfers into Level 3	36.1	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	(0.4)	0.0
Included in other comprehensive income	(4.8)	(0.8)
Purchases, issuances, sales, and settlements:
Purchases	9.0	0.1
Issuances	0.0	0.0
Sales	(2.0)	(3.5)
Settlements	0.0	0.0
Ending balance	40.7	2.8
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	0.0	0.0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		14.7
Beginning balance	13.3	17.5
Transfers into Level 3	1.5	1.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	(0.7)	4.2
Included in other comprehensive income	0.0	0.0
Purchases, issuances, sales, and settlements:
Purchases	1.0	1.2
Issuances	0.0	0.0
Sales	(1.0)	(10.6)
Settlements	0.0	0.0
Ending balance	15.5	13.3
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	$ (4.4)	$ 0.0