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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Income tax provision at expected rate	$ 10.8	$ 7.6	$ 18.0	$ 13.4
Tax-exempt interest	(0.7)	(0.9)	(1.5)	(1.8)
Dividends received deduction	(0.4)	(0.8)	(0.9)	(1.3)
Valuation allowance change	(1.0)	0.2	(1.1)	1.2
Other permanent adjustments, net	(1.1)	0.0	(1.0)	0.2
Adjustment for prior period	0.0	0.0	(0.7)	0.0
Adjustment for annualized rate	(1.8)	0.9	(2.5)	(0.6)
Other foreign adjustments	(0.4)	(0.6)	0.4	(0.6)
Deferred tax rate reduction	0.0	0.0	0.0	(0.3)
State income tax	0.1	0.0	0.1	0.0
Foreign exchange adjustments	(0.9)	2.1	(0.3)	3.6
Foreign withholding taxes	0.2	0.2	0.3	0.3
Income tax provision	4.8	8.7	10.8	14.1
Income tax provision - Foreign	(3.3)	0.6	(3.8)	0.4
Income tax provision - United States, Federal	7.8	7.9	14.2	13.5
Income tax provision - United States, State	0.1	0.0	0.1	0.0
Foreign withholding tax - United States	$ 0.2	$ 0.2	$ 0.3	$ 0.2