Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 5.6	$ (6.5)	$ 28.4	$ 2.8
Tax effect of:
Nontaxable investment income	(0.1)	(0.2)	(0.4)	(0.5)
Foreign exchange adjustments	0.2	0.7	(0.3)	0.1
Goodwill	0.0	0.0	0.0	1.0
Withholding taxes	0.0	0.0	0.1	0.1
Prior period adjustments	0.0	0.9	0.0	0.9
Change in uncertain tax position liability	(0.4)	0.0	(2.3)	0.0
Change in valuation allowance	0.8	1.2	(1.1)	1.2
Impact of change in tax rate related to Finance Act 2021	0.0	0.0	(7.4)	0.0
Other	(0.3)	0.5	(0.9)	1.3
Income tax provision (benefit)	$ 5.8	$ (3.4)	$ 16.1	$ 6.9