Prior Period Consolidated Financial Statements - Consolidated Statement of Changes in Stockholders' Equity (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
		Mar. 31, 2023	Mar. 31, 2022	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
Cash dividends (in usd per share)		$ 0.33	$ 0.32
Increase (Decrease) in Stockholders' Equity
Beginning balance		$ 1,098.3
Net income (loss)		6.6	$ 20.3
Change in net reserve remeasurements attributable to discount rates		(41.2)	181.5
Ending balance		1,139.2	1,351.5
Previously Reported
Increase (Decrease) in Stockholders' Equity
Beginning balance		1,088.2
Net income (loss)			14.5
Change in net reserve remeasurements attributable to discount rates			0.0
Ending balance			1,536.8
Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance		10.1
Net income (loss)			5.8
Change in net reserve remeasurements attributable to discount rates			181.5
Ending balance			(185.3)
Common stock, $0.001 par value
Increase (Decrease) in Stockholders' Equity
Beginning balance		0.1	0.1
Ending balance		0.1	0.1
Common stock, $0.001 par value | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			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
Additional paid-in capital
Increase (Decrease) in Stockholders' Equity
Beginning balance		502.6	495.3
Ending balance		503.1	496.6
Additional paid-in capital | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			496.6
Additional paid-in capital | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Ending balance			0.0
Retained earnings
Increase (Decrease) in Stockholders' Equity
Beginning balance		1,512.4	1,547.0
Net income (loss)		6.6	20.3
Dividends		(13.8)	(13.5)
Ending balance		1,505.2	1,553.0
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			1,524.9
Net income (loss)			14.5
Dividends			(13.5)
Ending balance			1,525.9
Retained earnings | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance			22.1
Net income (loss)			5.8
Dividends			0.0
Ending balance			27.1
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)
AOCI
Increase (Decrease) in Stockholders' Equity
Beginning balance		(399.4)	(50.0)
Change in net unrealized investment losses on fixed maturity securities		93.2	(334.1)
Change in net reserve remeasurements attributable to discount rates		(41.2)	181.5
Ending balance		(347.4)	(202.6)
AOCI | Previously Reported
Increase (Decrease) in Stockholders' Equity
Beginning balance			280.5
Change in net unrealized investment losses on fixed maturity securities			(270.7)
Change in net reserve remeasurements attributable to discount rates			0.0
Ending balance			9.8
AOCI | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Beginning balance			(330.5)
Change in net unrealized investment losses on fixed maturity securities			(63.4)
Change in net reserve remeasurements attributable to discount rates			181.5
Ending balance			(212.4)
Treasury stock, at cost
Increase (Decrease) in Stockholders' Equity
Beginning balance		(517.4)	(493.4)
Ending balance		$ (521.8)	(495.6)
Treasury stock, at cost | Previously Reported
Increase (Decrease) in Stockholders' Equity
Ending balance			(495.6)
Treasury stock, at cost | Effect of the Adoption of ASU 2018-12
Increase (Decrease) in Stockholders' Equity
Ending balance			$ 0.0

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