Other Comprehensive Income and Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	$ 2,788.4
Equity, Attributable to Parent, Ending Balance	2,917.6
Net Unrealized Losses on Fixed Maturities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	(687.8)	$ (530.9)
Other Comprehensive Income (Loss) Before Reclassifications	53.2	(96.1)
Reclassification from AOCI, Current Period, Tax	(0.4)	(2.6)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1.5	9.8
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(14.5)	22.7
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	54.7	(86.3)
Equity, Attributable to Parent, Ending Balance	(633.1)	(617.2)
Net Unrealized Losses on Investments with an Allowance for Credit Losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	(3.2)	(2.5)
Other Comprehensive Income (Loss) Before Reclassifications	2.3	1.6
Reclassification from AOCI, Current Period, Tax	0.3	0.2
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(1.3)	(0.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.3)	0.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1.0	1.4
Equity, Attributable to Parent, Ending Balance	(2.2)	(1.1)
Net Unrecognized Postretirement Benefit Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	8.4	9.5
Other Comprehensive Income (Loss) Before Reclassifications	0.0	0.0
Reclassification from AOCI, Current Period, Tax	0.1	0.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.4)	(0.8)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	0.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.4)	(0.8)
Equity, Attributable to Parent, Ending Balance	8.0	8.7
(Loss) Gain on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	(2.2)	2.5
Other Comprehensive Income (Loss) Before Reclassifications	2.4	0.0
Reclassification from AOCI, Current Period, Tax	0.1	(0.0)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.2)	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	(0.0)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	2.2	0.0
Equity, Attributable to Parent, Ending Balance	0.0	2.5
Change in Discount Rate on Future Life Policyholder Benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	380.3	160.6
Other Comprehensive Income (Loss) Before Reclassifications	(14.3)	111.5
Reclassification from AOCI, Current Period, Tax	(0.0)	(0.0)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	3.8	(29.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(14.3)	111.5
Equity, Attributable to Parent, Ending Balance	366.0	272.1
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Equity, Attributable to Parent, Beginning Balance	(304.5)	(360.8)
Other Comprehensive Income (Loss) Before Reclassifications	43.6	17.0
Reclassification from AOCI, Current Period, Tax	0.1	(2.3)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.4)	8.8
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(10.8)	(6.7)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	43.2	25.8
Equity, Attributable to Parent, Ending Balance	$ (261.3)	$ (335.0)