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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 9.8	$ 3.7	$ 23.2	$ 21.1
Tax-exempt interest	(0.7)	(1.2)	(2.5)	(3.1)
Dividends received deduction	(0.4)	(0.6)	(1.7)	(1.8)
Valuation allowance change	(0.8)	(1.9)	0.4	(2.9)
Other permanent adjustments, net	0.1	0.1	0.3	0.3
Adjustment for prior year tax return	(0.8)	(0.6)	(0.8)	(0.6)
Adjustment for annualized rate	(0.9)	0.8	(1.5)	0.6
United States state tax (benefit) expense	0.0	0.0	0.0	(2.5)
Other foreign adjustments	(0.1)	0.1	(0.7)	0.2
Prior year foreign taxes recovered	0.0	0.0	0.0	(1.2)
Deferred tax rate reduction	0.0	0.0	(0.3)	0.0
Foreign exchange adjustments	0.8	0.5	4.4	0.9
Foreign withholding taxes	0.0	0.0	0.3	0.3
Income tax provision	7.0	0.9	21.1	11.3
Income tax provision (benefit) - Foreign	0.2	(1.2)	0.6	(2.0)
Income tax provision - United States, Federal	6.8	2.1	20.2	16.8
Income tax benefit - United States, State	0.0	0.0	0.0	(3.8)
Foreign withholding tax - United States	$ 0.0	$ 0.0	$ 0.3	$ 0.3