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, 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	1.5	0.0
Transfers out of Level 3	0.0	0.0
Included in net income	6.6	(5.9)
Included in other comprehensive income	0.1	(0.5)
Purchases	0.1	5.3
Issuances	0.0	0.0
Sales	(3.5)	0.0
Settlements	0.0	0.0
Ending balance	29.3	24.5
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	0.0	0.0
Corporate Bonds
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
Included in net income	0.1	0.0
Included in other comprehensive income	0.1	(0.5)
Purchases	0.1	0.1
Issuances	0.0	0.0
Sales	(3.5)	0.0
Settlements	0.0	0.0
Ending balance	3.8	7.0
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets 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	1.5	0.0
Transfers out of Level 3	0.0	0.0
Included in net income	6.5	(5.9)
Included in other comprehensive income	0.0	0.0
Purchases	0.0	5.2
Issuances	0.0	0.0
Sales	0.0	0.0
Settlements	0.0	0.0
Ending balance	25.5	17.5
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	0.0	0.0
Short-term 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
Included in net income	0.0
Included in other comprehensive income	0.0
Purchases	0.0
Issuances	0.0
Sales	0.0
Settlements	0.0
Ending balance	0.0	$ 0.0
Amount Of Gains Or Losses For Period Included In Net Income Attributable To Change In Unrealized Gains Or Losses Relating To Assets Held At End Of Period	$ 0.0