Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Reclassification from AOCI to Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense (Benefit)	$ 74.8	$ 84.4	[1]	$ 125.6	[1]
Net Loss	(272.1)	(286.6)	[1]	(123.7)	[1]
Stockholders' Equity Attributable to Parent	2,505.2	2,670.6	[2]
AOCI, Liability for Future Policy Benefit, after Tax						$ (1,030.3)
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(530.9)	(719.4)		505.8		730.6
OCI, before Reclassifications, Net of Tax, Attributable to Parent	185.0	(1,215.1)		(182.0)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	3.5	(10.1)		(42.8)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	188.5	(1,225.2)		(224.8)
Reclassification from AOCI, Current Period, Tax	(0.9)	2.7		11.3
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(50.3)	325.9		59.7
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(2.5)	(2.2)		(3.7)		(2.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(0.3)	2.0		(1.6)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	(0.5)		0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.3)	1.5		(1.6)
Reclassification from AOCI, Current Period, Tax	0.0	0.1		0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.2	(0.4)		0.4
Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	9.5	(37.2)		(52.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(6.0)	15.2		(6.5)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax						(45.7)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	52.7	(0.3)		0.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	46.7	14.9		(6.4)
Reclassification from AOCI, Current Period, Tax	(13.8)	0.1		0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(12.5)	(4.0)		2.4
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	2.5	2.8		(1.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	4.7		0.0
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax						(2.3)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.3)	0.0		0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.3)	4.7		0.4
Reclassification from AOCI, Current Period, Tax	(0.1)	0.0		(0.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.1)	(1.2)		(0.1)
Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(360.8)	(514.9)		(401.6)		$ (349.8)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	98.2	(102.4)		(9.5)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	55.9	(10.9)		(42.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	154.1	(113.3)		(51.8)
Reclassification from AOCI, Current Period, Tax	(14.8)	2.9		11.2
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(41.5)	30.4		14.5
Accumulated Liability For Future Policy Benefit Adjustment Attributable to Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	160.6	241.1		(849.7)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(80.5)	1,090.8		180.6
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0		0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(80.5)	1,090.8		180.6
Reclassification from AOCI, Current Period, Tax	0.0	0.0		0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	$ 21.2	$ (289.9)		$ (47.9)

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.

[2] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.