Schedule of deferred income tax assets and liabilities (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax [Abstract]
Deferred Acquisition Cost	$ 97.4	$ 70.0
Deferred Tax Liabilities, Property, Plant and Equipment	93.0	80.3
Deferred Tax Liabilities, Investments	1,999.2	1,274.3
Other	64.6us-gaap_DeferredTaxLiabilitiesOther	54.4us-gaap_DeferredTaxLiabilitiesOther
Gross Deferred Tax Liability	2,254.2us-gaap_DeferredTaxLiabilities	1,479.0us-gaap_DeferredTaxLiabilities
Invested Assets	1,919.2	1,180.1
Employee Benefits	254.0	151.2
Other	2.6us-gaap_DeferredTaxAssetsOther	3.4us-gaap_DeferredTaxAssetsOther
Gross Deferred Tax Asset	2,175.8us-gaap_DeferredTaxAssetsGross	1,334.7us-gaap_DeferredTaxAssetsGross
Total Net Deferred Tax Liability	$ 78.4us-gaap_DeferredIncomeTaxLiabilities	$ 144.3us-gaap_DeferredIncomeTaxLiabilities

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Amount of deferred tax liability attributable to taxable temporary differences.

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-Name Accounting Standards Codification

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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The tax effect as of the balance sheet date of the amount of the estimated future tax deductions arising from employee benefit costs not otherwise specified in the taxonomy, which can only be deducted for tax purposes when actual costs are incurred, and which can only be realized if sufficient tax-basis income is generated in future periods to enable the deduction to be taken.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from unrealized losses on available-for-sale securities.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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The amount as of the balance sheet date of the estimated future tax effects attributable to policy acquisition costs being expensed for tax purposes but capitalized in conformity with generally accepted accounting principles, which will reverse in future periods when amortization of such capitalized costs cannot be deducted for tax purposes.

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Amount of deferred tax liability attributable to taxable temporary differences from investments (excludes investments in subsidiaries and equity method investments).

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Amount of deferred tax liability attributable to taxable temporary differences from property, plant, and equipment.

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