Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2022	Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 17.5	$ 24.5
Transfers into Level 3	43.2	2.4
Transfers out of Level 3	0.0	0.0
Included in net income	0.3	4.2
Included in other comprehensive income	(2.9)	(0.8)
Purchases	2.5	1.3
Issuances	0.0	0.0
Sales	(1.8)	(14.1)
Settlements	0.0	0.0
Ending balance	58.8	17.5
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	(3.6)	0.0
Credit Financial
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2.8	7.0
Transfers into Level 3	42.6	0.0
Transfers out of Level 3	0.0	0.0
Included in net income	(0.3)	0.0
Included in other comprehensive income	(2.9)	(0.8)
Purchases	1.4	0.1
Issuances	0.0	0.0
Sales	(1.8)	(3.5)
Settlements	0.0	0.0
Ending balance	41.8	2.8
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	0.0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	14.7	17.5
Transfers into Level 3	0.6	2.4
Transfers out of Level 3	0.0	0.0
Included in net income	0.6	4.2
Included in other comprehensive income	0.0	0.0
Purchases	1.1	1.2
Issuances	0.0	0.0
Sales	0.0	(10.6)
Settlements	0.0	0.0
Ending balance	17.0	14.7
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	$ (3.6)	$ 0.0