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, 2020	Mar. 31, 2019
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate	$ (30.2)	$ 12.9
Tax-exempt interest and dividends received deduction	(0.7)	(0.7)
Other, net	1.5	0.0
Federal income tax expense and effective rate	(29.4)	12.2
Income Tax Expense (Benefit)	$ (29.4)	$ 12.2
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Amount at statutory rate (percent)	21.00%	21.00%
Tax-exempt interest and dividends received deduction (percent)	0.50%	(1.10%)
Other, net (percent)	(1.10%)	0.00%
Effective Income Tax Rate Reconciliation, Deduction, Percent	20.40%	19.90%