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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 12.7	$ 34.1	$ 25.8
Nontaxable investment income	(4.7)	(5.5)	(6.5)
Foreign exchange adjustments	2.1	5.3	(0.1)
Withholding taxes	0.4	2.4	2.9
Change in valuation allowance	(0.9)	0.7	(2.6)
Impact of change in tax rate related to TCJA	(20.2)	0.0	0.0
Other	0.2	(1.8)	(5.2)
Income tax (benefit) provision	$ (10.4)	$ 35.2	$ 14.3