Income Taxes - Reconciliation of Difference Between Provision for Income Taxes and Expected Tax Provision at Weighted Average Tax Rate (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Examination [Line Items]
Income tax provision at expected rate	$ 18.5	$ 26.0	$ 42.6
Tax-exempt interest	(5.6)	(5.8)	(6.1)
Dividends received deduction	(2.2)	(1.6)	(1.4)
Valuation allowance change	2.1	0.8	(1.0)
Other permanent adjustments, net	1.8	0.3	0.9
Adjustment for prior year tax return	0.2	1.6	1.2
United States state tax benefit	(0.3)	(0.3)	0
Other foreign adjustments	0	0.1	0.2
Prior year foreign taxes recovered	0.6	(0.6)	(0.7)
Rate adjustment on loss carrybacks	0	(0.4)	0
Deferred tax rate reduction	(0.6)	(0.4)	(0.1)
Foreign exchange adjustments	0.5	0.3	(0.4)
Income tax provision	14.4	20.0	35.2
Income tax provision	14.4	20.0	35.2
Foreign Country [Member]
Income Tax Examination [Line Items]
Income tax provision	3.3	(8.0)	(2.2)
Income tax provision	3.3	(8.0)	(2.2)
United States Federal [Member]
Income Tax Examination [Line Items]
Income tax provision	11.5	28.5	37.3
Income tax provision	11.5	28.5	37.3
United States State [Member]
Income Tax Examination [Line Items]
Income tax provision	(0.4)	(0.5)	0.1
Income tax provision	$ (0.4)	$ (0.5)	$ 0.1