Federal Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 6.7	$ 23.5	$ 9.4
Tax-exempt interest and dividends received deduction	(7.1)	(8.7)	(8.5)
Other, net	(1.4)	1.3	1.1
Valuation allowance	0.0	0.0	(82.6)
Federal income tax (benefit) expense and effective rate	$ (1.8)	$ 16.1	$ (80.6)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	(37.20%)	(13.00%)	(31.50%)
Other, net (percent)	(7.20%)	1.90%	4.10%
Valuation allowance (percent)	0.00%	0.00%	(308.10%)
Federal income tax expense (benefit) and effective rate (percent)	(9.40%)	23.90%	(300.50%)