Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ (1.8)	$ 1.3
Tax-exempt interest and dividends received deduction	(1.5)	(2.0)
Other, net	2.2	1.5
Federal income tax expense and effective rate	$ (1.1)	$ 0.8
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)	29.10%	(52.80%)
Other, net (percent)	(42.40%)	37.80%
Federal income tax expense (benefit) and effective rate (percent)	21.70%	20.00%