Prior Period Consolidated Financial Statements - Consolidated Statement of Changes in Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended										12 Months Ended
		Dec. 31, 2022		Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021		Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Common stock, par value (in usd per share)		$ 0.001					$ 0.001					$ 0.001	$ 0.001		$ 0.001
Cash dividends (in usd per share)												$ 1.32	$ 1.28		$ 1.24
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					$ 1,499.0						$ 1,098.3	$ 1,499.0
Net income		$ (16.7)		$ 20.4	$ (4.2)	20.3	$ 46.9		$ 20.2	$ 54.2	$ 49.1	45.0	19.8	[2],[3]	$ 170.4	[2],[3]
Change in net reserve remeasurements attributable to discount rates												(37.1)	445.9	[2]	110.8	[2]
Change in net funded status of benefit plans													1.4
Ending balance		1,098.3	[1]				1,499.0	[1]				1,175.3	1,098.3	[1]	1,499.0	[1]
Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						1,807.4						1,088.2	1,807.4
Net income													(2.6)		142.8
Change in net reserve remeasurements attributable to discount rates													0.0		0.0
Change in net funded status of benefit plans													1.4
Ending balance		1,088.2					1,807.4						1,088.2		1,807.4
Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(308.4)						10.1	(308.4)
Net income													22.4		27.6
Change in net reserve remeasurements attributable to discount rates													445.9		110.8
Change in net funded status of benefit plans													0.0
Ending balance		10.1					(308.4)						10.1		(308.4)
Common stock, $0.001 par value
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					0.1					0.1	0.1	0.1		0.1
Ending balance		0.1	[1]				0.1	[1]				0.1	0.1	[1]	0.1	[1]
Common stock, $0.001 par value | Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						0.1						0.1	0.1
Ending balance		0.1					0.1						0.1		0.1
Common stock, $0.001 par value | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						0.0						0.0	0.0
Ending balance		0.0					0.0						0.0		0.0
Additional paid-in capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					495.3					488.3	502.6	495.3		488.3
Ending balance		502.6	[1]				495.3	[1]				510.9	502.6	[1]	495.3	[1]
Additional paid-in capital | Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						495.3						502.6	495.3
Ending balance		502.6					495.3						502.6		495.3
Additional paid-in capital | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						0.0						0.0	0.0
Ending balance		0.0					0.0						0.0		0.0
Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					1,547.0					1,434.6	1,512.4	1,547.0		1,434.6
Net income												45.0	19.8	[1]	170.4	[1]
Dividends,per share; 2022, $0.32 per share												(55.2)	(53.7)	[1]	(52.5)	[1]
Ending balance		1,512.4	[1]				1,547.0	[1]				1,502.2	1,512.4	[1]	1,547.0	[1]
Retained earnings | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(0.7)					(5.5)		(0.7)		(5.5)
Ending balance							(0.7)								(0.7)
Retained earnings | Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						1,524.9					1,434.6	1,468.6	1,524.9		1,434.6
Net income													(2.6)		142.8
Dividends,per share; 2022, $0.32 per share													(53.7)		(52.5)
Ending balance		1,468.6					1,524.9						1,468.6		1,524.9
Retained earnings | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						22.1					0.0	43.8	22.1		0.0
Net income													22.4		27.6
Dividends,per share; 2022, $0.32 per share													0.0		0.0
Ending balance		43.8					22.1						43.8		22.1
Retained earnings | Effect of the Adoption of ASU 2018-12 | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(0.7)					(5.5)		(0.7)		(5.5)
Ending balance							(0.7)								(0.7)
Accumulated other comprehensive income (loss), net of tax:
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					(50.0)					355.1	(399.4)	(50.0)		355.1
Change in net unrealized investment gains (losses) on fixed maturity securities													(796.7)		(90.3)
Change in net reserve remeasurements attributable to discount rates												(37.1)	445.9	[1]	110.8	[1]
Ending balance		(399.4)	[1]				(50.0)	[1]				(314.0)	(399.4)	[1]	(50.0)	[1]
Accumulated other comprehensive income (loss), net of tax: | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance											(426.6)				(426.6)
Accumulated other comprehensive income (loss), net of tax: | Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						280.5					355.1	(365.7)	280.5		355.1
Change in net unrealized investment gains (losses) on fixed maturity securities													(647.6)		(75.6)
Change in net reserve remeasurements attributable to discount rates													0.0		0.0
Ending balance		(365.7)					280.5						(365.7)		280.5
Accumulated other comprehensive income (loss), net of tax: | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(330.5)					0.0	(33.7)	(330.5)		0.0
Change in net unrealized investment gains (losses) on fixed maturity securities													(149.1)		(14.7)
Change in net reserve remeasurements attributable to discount rates													445.9		110.8
Ending balance		(33.7)					(330.5)						(33.7)		(330.5)
Accumulated other comprehensive income (loss), net of tax: | Effect of the Adoption of ASU 2018-12 | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance											(426.6)				(426.6)
Treasury stock, at cost
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance	[1]					(493.4)					$ (488.1)	(517.4)	(493.4)		(488.1)
Ending balance		(517.4)	[1]				(493.4)	[1]				(523.9)	(517.4)	[1]	(493.4)	[1]
Treasury stock, at cost | Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						(493.4)						(517.4)	(493.4)
Ending balance		(517.4)					(493.4)						(517.4)		(493.4)
Treasury stock, at cost | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance						$ 0.0						$ 0.0	0.0
Ending balance		$ 0.0					$ 0.0						$ 0.0		$ 0.0

[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

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