Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax provision (benefit) at expected rate	$ 9.0	$ 8.2	$ 12.7
Tax effect of:
Nontaxable investment income	(1.2)	(1.9)	(4.7)
Foreign exchange adjustments	(0.1)	(0.6)	2.1
Impairment of goodwill	2.9	0.0	0.0
Withholding taxes	0.2	0.4	0.4
Change in valuation allowance	(1.1)	(1.5)	(0.9)
Impact of change in tax rate related to TCJA	0.0	(1.6)	(20.2)
Other	(1.1)	1.1	0.2
Income tax provision (benefit)	$ 8.6	$ 4.1	$ (10.4)