Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 24.5	$ 25.6
Transfers into Level 3	1.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	4.2	(5.9)
Included in other comprehensive income	(0.8)	(0.5)
Purchases, issuances, sales, and settlements:
Purchases	1.3	5.3
Issuances	0.0	0.0
Sales	(14.1)	0.0
Settlements	0.0	0.0
Ending balance	16.1	24.5
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
Credit Financial
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7.0	7.4
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	0.0	0.0
Included in other comprehensive income	(0.8)	(0.5)
Purchases, issuances, sales, and settlements:
Purchases	0.1	0.1
Issuances	0.0	0.0
Sales	(3.5)	0.0
Settlements	0.0	0.0
Ending balance	2.8	7.0
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	0.0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	17.5	18.2
Transfers into Level 3	1.0	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	4.2	(5.9)
Included in other comprehensive income	0.0	0.0
Purchases, issuances, sales, and settlements:
Purchases	1.2	5.2
Issuances	0.0	0.0
Sales	(10.6)	0.0
Settlements	0.0	0.0
Ending balance	13.3	17.5
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