Federal Income Taxes - Reconciliation Between Actual Federal Income Tax Expense (Benefit) and Amount Computed at Indicated Statutory Rate (Details) - USD ($) $ in Millions		12 Months Ended
	Dec. 23, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Amount at statutory rate		$ 2.7	$ 12.3	$ 6.6
Tax-exempt interest and dividends received deduction		(2.3)	(5.7)	(7.1)
Other, net		(0.3)	2.7	0.1
Federal income tax expense (benefit)		0.1	9.3	(0.4)
Impact of TCJA at enactment		0.0	43.5	0.0
Total federal income tax expense (benefit)		$ 0.1	$ 52.8	$ (0.4)
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	21.00%	35.00%	35.00%
Tax-exempt interest and dividends received deduction (percent)		(17.70%)	(16.30%)	(37.60%)
Other, net (percent)		(2.80%)	7.70%	0.30%
Effective Income Tax Rate Reconciliation, Deduction, Percent		0.50%	26.40%	(2.30%)
Impact of TCJA at enactment, Percent		0.00%		0.00%
Federal income tax expense (benefit) and effective rate (percent)		0.50%	150.50%	(2.30%)