Long-Duration Insurance Contracts - Deferred Policy Acquisition Costs, Rollforward (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period	[1]	$ 330.6
Amortization expense		(101.2)	$ (88.2)	[2]	$ (90.6)	[2]
Balance, end of period		336.3	330.6	[1]
Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		433.1
Amortization expense			(98.7)		(94.7)
Balance, end of period			433.1
Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		(102.5)
Amortization expense			10.5		4.1
Balance, end of period			(102.5)
Whole Life
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		20.9	19.1		17.8
Capitalizations		2.7	3.0		2.4
Amortization expense		(1.2)	(1.2)		(1.1)
Experience adjustment		(0.1)	0.0		0.0
Balance, end of period		22.3	20.9		19.1
Whole Life | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					17.8
Whole Life | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					0.0
Term Life
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		30.0	27.5		25.6
Capitalizations		5.8	5.0		4.2
Amortization expense		(3.1)	(2.5)		(2.3)
Experience adjustment		(0.1)	0.0		0.0
Balance, end of period		32.6	30.0		27.5
Term Life | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					25.6
Term Life | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					0.0
Experience life
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		5.8	6.0		6.2
Capitalizations		0.3	0.2		0.2
Amortization expense		(0.4)	(0.4)		(0.4)
Experience adjustment		0.0	0.0		0.0
Balance, end of period		5.7	5.8		6.0
Experience life | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					2.6
Experience life | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					3.6
Limited-pay whole life
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		6.7	5.6		4.4
Capitalizations		1.1	1.4		1.5
Amortization expense		(0.3)	(0.3)		(0.3)
Experience adjustment		(0.1)	0.0		0.0
Balance, end of period		7.4	6.7		5.6
Limited-pay whole life | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					4.4
Limited-pay whole life | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					0.0
Indexed Universal Life
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		15.4	13.7		12.9
Capitalizations		2.5	2.5		1.7
Amortization expense		(1.0)	(0.8)		(0.8)
Experience adjustment		(0.1)	0.0		(0.1)
Balance, end of period		16.8	15.4		13.7
Indexed Universal Life | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					11.3
Indexed Universal Life | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					1.6
Supplemental health
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		6.2	4.9		4.3
Capitalizations		2.9	1.8		1.1
Amortization expense		(0.6)	(0.5)		(0.5)
Experience adjustment		(0.3)	0.0		0.0
Balance, end of period		8.2	6.2		4.9
Supplemental health | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					4.3
Supplemental health | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					0.0
Total Annuities
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period		221.1	223.3		223.1
Capitalizations		15.0	15.5		17.3
Amortization expense		(14.7)	(15.8)		(16.0)
Experience adjustment		(7.4)	(1.9)		(1.1)
Balance, end of period		$ 214.0	$ 221.1		223.3
Total Annuities | Previously Reported
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					137.7
Total Annuities | Effect of the Adoption of ASU 2018-12
Movement Analysis of Deferred Policy Acquisition Costs [Roll Forward]
Balance, beginning of period					$ 85.4

[1] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

[2] Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements