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, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ 23.5	$ 9.4	$ 21.5
Tax-exempt interest and dividends received deduction	(8.7)	(8.5)	(9.4)
Other, net	1.3	1.1	0.2
Valuation allowance	0.0	(82.6)	(11.8)
Federal income tax expense (benefit) and effective rate	$ 16.1	$ (80.6)	$ 0.5
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)	(13.00%)	(31.50%)	(15.30%)
Other, net (percent)	1.90%	4.10%	0.40%
Valuation allowance (percent)	0.00%	(308.10%)	(19.30%)
Federal income tax expense (benefit) and effective rate (percent)	23.90%	(300.50%)	0.80%