Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ (1.6)	$ 1.6	$ 11.3	$ 0.9
Tax-exempt interest and dividends received deduction	(0.7)	(0.5)	(1.4)	(1.2)
Other, net	0.9	0.6	0.9	0.8
Federal income tax expense and effective rate	(1.4)	1.7	10.8	0.5
Income Tax Expense (Benefit)	$ (1.4)	$ 1.7	$ 10.8	$ 0.5
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	21.00%	21.00%	21.00%	21.00%
Tax-exempt interest and dividends received deduction (percent)	9.00%	(6.50%)	(2.60%)	(27.20%)
Other, net (percent)	(10.70%)	7.20%	1.50%	17.00%
Effective Income Tax Rate Reconciliation, Deduction, Percent	19.30%	21.70%	19.90%	10.80%

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 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 deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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

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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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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 other adjustments.

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