Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 11.6	$ 9.7
Tax-exempt interest and dividends received deduction	(2.2)	(2.1)
Other, net	(0.9)	0.2
Valuation allowance	0	(7.2)
Federal income tax expense and effective rate	$ 8.5	$ 0.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(7.00%)	(8.00%)
Other, net (percent)	(3.00%)	1.00%
Valuation allowance (percent)	0.00%	(26.00%)
Federal income tax expense (benefit) and effective rate (percent)	25.00%	2.00%

Income tax reconciliation tax exempt income and dividend deduction.

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Income tax reconciliation tax exempt income and dividend deductions tax rate.

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Amount of current and deferred federal income tax expense (benefit) attributable to income (loss) from continuing operations.

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-Glossary Income Tax Expense (or Benefit)

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to increase (decrease) in the valuation allowance for deferred tax assets.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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