Income Taxes - Temporary Differences (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Insurance reserves	$ 125.0	$ 224.0
Investments	75.0	141.0
Compensation and benefits	55.0	71.0
Other assets	3.0	8.0
Total gross assets before valuation allowance	258.0	444.0
Valuation allowance, deferred tax assets	0.0	5.0
Assets, net of valuation allowance	258.0	439.0
Net unrealized investment (gains) losses	(311.0)	(346.0)
Deferred policy acquisition costs	(134.0)	(316.0)
Total gross liabilities	(445.0)	(662.0)
Net deferred income tax asset (liability)	$ (187.0)	$ (223.0)

Amount of deferred tax liability attributable to taxable temporary differences.

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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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The tax effect as of the balance sheet date of the amount of the estimated future tax deductions arising from estimated policyholder reserves, which will be deductible from future taxable income when actual costs are incurred, and which can only be realized if sufficient tax-basis income is generated in future periods to enable the tax deduction to be taken.

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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