Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 8.6	$ (3.9)	$ 9.5	$ (1.2)
Tax-exempt interest and dividends received deduction	(0.5)	(1.4)	(1.7)	(4.4)
Other, net	(0.5)	3.6	0.3	5.1
Federal income tax expense and effective rate	7.6	(1.7)	8.1	(0.5)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	0.0	0.0	0.0	1.3
Income Tax Expense (Benefit)	$ 7.6	$ (1.7)	$ 8.1	$ 0.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	21.00%	35.00%	21.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(1.30%)	12.40%	(3.80%)	131.80%
Other, net (percent)	(1.10%)	(32.60%)	0.60%	(151.70%)
Effective Income Tax Rate Reconciliation, Deduction, Percent	18.60%	14.80%	17.80%	15.10%