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, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Examination [Line Items]
Income tax provision at expected rate	$ 37.2	$ 43.1	$ 18.5
Tax-exempt interest	(4.5)	(5.6)	(5.6)
Dividends received deduction	(2.3)	(2.3)	(2.2)
Valuation allowance change	(30.8)	3.7	2.1
Other permanent adjustments, net	(0.7)	0.3	1.8
Adjustment for prior year tax return	(0.9)	(1.6)	0.2
United States state tax expense (benefit)	2.5	0.3	(0.3)
PXRE Reinsurance capital loss carryforward	29.8
Other foreign adjustments	0.7	(0.1)
Deferred tax rate reduction	(0.4)	(1.2)	(0.6)
Foreign exchange adjustments	0.2	(1.7)	0.5
Foreign withholding taxes	0.4	0.5
Income tax provision	32.8	36.5	14.4
Income tax provision - Foreign	5.2	2.4	3.3
Income tax provision - United States, Federal	23.3	33.2	11.5
Income tax provision (benefit) - United States, state	3.9	0.4	(0.4)
Foreign withholding tax - United States	0.4	0.5
Income tax provision	32.8	36.5	14.4
Scenario, Previously Reported [Member]
Income Tax Examination [Line Items]
Prior period adjustment for deferred	1.5
Prior year foreign taxes	$ 0.1	$ 1.1