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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income tax (benefit) provision at expected rate	$ 18.8	$ 10.8	$ 18.4	$ 18.0
Tax effect of:
Nontaxable investment income	(0.5)	(1.1)	(1.0)	(2.4)
Foreign exchange adjustments	0.0	(0.9)	(0.2)	(0.3)
Withholding taxes	0.2	0.2	0.3	0.3
Change in valuation allowance	(0.7)	(1.0)	(3.4)	(1.1)
Other	(2.5)	(3.2)	1.4	(3.7)
Income tax provision	$ 15.3	$ 4.8	$ 15.5	$ 10.8