Other Comprehensive Income and Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Stockholders' Equity Attributable to Parent	$ 2,773.3		$ 2,773.3		$ 358.0	$ 2,505.2
Net Unrealized Losses on Fixed Maturities
Stockholders' Equity Attributable to Parent	(453.1)	$ (896.2)	(453.1)	$ (896.2)	(679.6)	(530.9)	$ (637.6)	$ (719.4)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	226.4	(258.7)	68.9	(183.3)
Reclassification from AOCI, Current Period, Tax	0.0	0.1	(2.3)	(1.7)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.1	0.1	8.9	6.5
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(60.1)	67.5	(20.6)	47.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	226.5	(258.6)	77.8	(176.8)
Net Unrealized Losses on Investments with an Allowance for Credit Losses
Stockholders' Equity Attributable to Parent	(3.5)	(3.4)	(3.5)	(3.4)	(3.7)	(2.5)	(2.3)	(2.2)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.2	(0.9)	(0.5)	0.5
Reclassification from AOCI, Current Period, Tax	0.0	0.1	0.2	0.5
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	(0.2)	(0.5)	(1.7)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.2	0.8	0.7	0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	(1.1)	(1.0)	(1.2)
Net Unrecognized Postretirement Benefit Income
Stockholders' Equity Attributable to Parent	7.7	10.5	7.7	10.5	8.3	9.5	(38.0)	(37.2)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	(5.4)	0.0	(5.4)
Reclassification from AOCI, Current Period, Tax	0.1	(14.4)	0.6	(14.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.6)	53.9	(1.8)	53.1
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	(12.6)	0.6	(12.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.6)	48.5	(1.8)	47.7
(Loss) Gain on Cash Flow Hedges
Stockholders' Equity Attributable to Parent	3.6	2.7	3.6	2.7	(0.3)	2.5	2.7	2.8
OCI, before Reclassifications, Net of Tax, Attributable to Parent	3.5	0.0	0.7	0.0
Reclassification from AOCI, Current Period, Tax	(0.1)	0.0	(0.1)	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.4	0.0	0.4	(0.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(1.0)	0.0	(0.2)	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	3.9	0.0	1.1	(0.1)
Change in Discount Rate on Future Life Policyholder Benefits
Stockholders' Equity Attributable to Parent	192.8	413.0	192.8	413.0		160.6	194.4	241.1
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(165.2)	218.6	32.2	171.9
Reclassification from AOCI, Current Period, Tax	0.0	0.0	0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	43.9	(58.2)	(8.6)	(45.7)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(165.2)	218.6	32.2	171.9
Accumulated Other Comprehensive (Loss) Income
Stockholders' Equity Attributable to Parent	(252.5)	(473.4)	(252.5)	(473.4)	$ (317.3)	$ (360.8)	$ (480.8)	$ (514.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	64.9	(46.4)	101.3	(16.3)
Reclassification from AOCI, Current Period, Tax	0.0	(14.2)	(1.6)	(15.3)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.1)	53.8	7.0	57.8
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(16.9)	(2.5)	(28.1)	(10.8)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 64.8	$ 7.4	$ 108.3	$ 41.5