Investments - Schedule of Reconciliation of Beginning and Ending Balances for Investments Categorized as Level 3 (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 10.4	$ 4.2
Transfers into Level 3	3.5	0.0
Transfers out of Level 3	0.0	0.0
Total gains or losses (realized/unrealized):
Included in net income	(2.0)	0.2
Included in other comprehensive income	0.6	0.3
Purchases, issuances, sales, and settlements:
Purchases	1.9	7.3
Issuances	0.0	0.0
Sales	(0.4)	(1.6)
Settlements	0.0	0.0
Ending balance	14.0	10.4
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	0.0
Credit-Financial [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2.2	1.9
Transfers into Level 3	3.5	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	0.6	0.3
Purchases, issuances, sales, and settlements:
Purchases	1.9	0.0
Issuances	0.0	0.0
Sales	(0.4)	0.0
Settlements	0.0	0.0
Ending balance	7.4	2.2
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 [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	8.2	2.3
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	(1.6)	0.2
Included in other comprehensive income	0.0	0.0
Purchases, issuances, sales, and settlements:
Purchases	0.0	7.3
Issuances	0.0	0.0
Sales	0.0	(1.6)
Settlements	0.0	0.0
Ending balance	6.6	8.2
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