Prior Period Consolidated Financial Statements - Consolidated Statement of Changes in Stockholders' Equity (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
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)			$ 0.32	$ 0.33	$ 0.32
Increase (Decrease) in Stockholders' Equity
Beginning balance				$ 1,098.3
Net income (loss)		$ 11.7	$ 20.4	5.5	$ 36.5
Change in net reserve remeasurements attributable to discount rates		90.7	110.7	74.6	460.7
Change in net funded status of benefit plans		0.0	0.0	0.0	0.0
Ending balance		1,051.3	1,092.3	1,051.3	1,092.3
Previously Reported
Increase (Decrease) in Stockholders' Equity
Beginning balance				1,088.2
Net income (loss)			13.9		15.9
Change in net reserve remeasurements attributable to discount rates			0.0		0.0
Change in net funded status of benefit plans			0.0		0.0
Ending balance			1,076.6		1,076.6
Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance				10.1
Net income (loss)			6.5		20.6
Change in net reserve remeasurements attributable to discount rates			110.7		460.7
Change in net funded status of benefit plans			0.0		0.0
Ending balance			15.7		15.7
Common stock, $0.001 par value
Increase (Decrease) in Stockholders' Equity
Beginning balance		0.1	0.1	0.1	0.1
Ending balance		0.1	0.1	0.1	0.1
Common stock, $0.001 par value | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			0.1		0.1
Common stock, $0.001 par value | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Ending balance			0.0		0.0
Additional paid-in capital
Increase (Decrease) in Stockholders' Equity
Beginning balance		505.7	498.1	502.6	495.3
Ending balance		508.5	500.4	508.5	500.4
Additional paid-in capital | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			500.4		500.4
Additional paid-in capital | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Ending balance			0.0		0.0
Retained earnings
Increase (Decrease) in Stockholders' Equity
Beginning balance		1,478.6	1,535.3	1,512.4	1,547.0
Net income (loss)		11.7	20.4	5.5	36.5
Dividends,per share; 2022, $0.32 per share		(13.8)	(13.3)	(41.4)	(40.3)
Ending balance		1,476.5	1,542.4	1,476.5	1,542.4
Retained earnings | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance					(0.8)
Retained earnings | Previously Reported
Increase (Decrease) in Stockholders' Equity
Beginning balance			1,499.9		1,524.9
Net income (loss)			13.9		15.9
Dividends,per share; 2022, $0.32 per share			(13.4)		(40.4)
Ending balance			1,500.4		1,500.4
Retained earnings | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance			35.4		22.1
Net income (loss)			6.5		20.6
Dividends,per share; 2022, $0.32 per share			0.1		0.1
Ending balance			42.0		42.0
Retained earnings | Effect of the Adoption of ASU 2018-12 | Accounting Standards Update 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance	[1]				(0.8)
Accumulated other comprehensive income (loss), net of tax:
Increase (Decrease) in Stockholders' Equity
Beginning balance		(359.6)	(328.9)	(399.4)	(50.0)
Change in net unrealized investment losses on fixed maturity securities		(141.0)	(215.0)	(85.1)	(843.9)
Change in net reserve remeasurements attributable to discount rates		90.7	110.7	74.6	460.7
Ending balance		(409.9)	(433.2)	(409.9)	(433.2)
Accumulated other comprehensive income (loss), net of tax: | Previously Reported
Increase (Decrease) in Stockholders' Equity
Beginning balance			(230.6)		280.5
Change in net unrealized investment losses on fixed maturity securities			(176.3)		(687.4)
Change in net reserve remeasurements attributable to discount rates			0.0		0.0
Ending balance			(406.9)		(406.9)
Accumulated other comprehensive income (loss), net of tax: | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance			(98.3)		(330.5)
Change in net unrealized investment losses on fixed maturity securities			(38.7)		(156.5)
Change in net reserve remeasurements attributable to discount rates			110.7		460.7
Ending balance			(26.3)		(26.3)
Treasury stock, at cost
Increase (Decrease) in Stockholders' Equity
Beginning balance		(522.9)	(507.4)	(517.4)	(493.4)
Ending balance		$ (523.9)	(517.4)	$ (523.9)	(517.4)
Treasury stock, at cost | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			(517.4)		(517.4)
Treasury stock, at cost | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Ending balance			$ 0.0		$ 0.0

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