Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Stockholders' Equity Attributable to Parent	$ 2,361.3		$ 2,361.3			$ 2,670.6
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax					$ (38.0)	(37.2)		$ (52.1)
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax					2.7	2.8		(1.9)
AOCI, Liability for Future Policy Benefit, Expected Future Policy Benefit, before Tax	241.1	$ (849.7)	241.1	$ (849.7)	194.4		$ 11.4
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Stockholders' Equity Attributable to Parent	(3.4)	(4.7)	(3.4)	(4.7)	(2.3)	(2.2)	(4.1)	(3.7)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(0.9)	(0.6)	0.5	(1.0)
Reclassification from AOCI, Current Period, Tax	0.1	0.0	0.5	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.2)	0.0	(1.7)	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.8	0.2	0.4	0.3
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(1.1)	(0.6)	(1.2)	(1.0)
Accumulated Other Comprehensive Loss
Stockholders' Equity Attributable to Parent	(473.4)	(573.8)	(473.4)	(573.8)	(480.8)	(514.9)	(509.4)	(401.6)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(46.4)	(48.3)	(16.3)	(154.8)
Reclassification from AOCI, Current Period, Tax	(14.2)		(15.3)	4.5
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	53.8	(16.1)	57.8	(17.4)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(2.5)	17.2	(10.8)	45.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	7.4	(64.4)	41.5	(172.2)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	10.5	(52.3)	10.5	(52.3)			52.2
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(5.4)	0.0		0.0
Reclassification from AOCI, Current Period, Tax	(14.4)	0.0	(14.1)	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	53.9	(0.1)	53.1	(0.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(12.6)	0.1	(12.6)	0.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	48.5	(0.1)	47.7	(0.2)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Stockholders' Equity Attributable to Parent	2.7	2.8	2.7	2.8			2.8
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	0.0	0.0	4.7
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	(0.1)	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.0	0.0	0.0	(1.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	0.0	(0.1)	4.7
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Stockholders' Equity Attributable to Parent	(896.2)	(792.4)	(896.2)	(792.4)	$ (637.6)	$ (719.4)	$ (467.3)	$ 505.8
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(258.7)	(309.1)	(183.3)	(1,281.0)
Reclassification from AOCI, Current Period, Tax	0.1		(1.7)	4.5
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.1	(16.0)	6.5	(17.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	67.5	86.4	47.1	345.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(258.6)	(325.1)	(176.8)	(1,298.2)
AOCI, Liability for Future Policy Benefit, Parent
Stockholders' Equity Attributable to Parent	413.0	272.8	413.0	272.8
OCI, before Reclassifications, Net of Tax, Attributable to Parent	218.6	261.4	171.9	1,122.5
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
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(58.2)	(69.5)	(45.7)	(298.4)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 218.6	$ 261.4	$ 171.9	$ 1,122.5