CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Effect of the Adoption of ASU 2018-12	Common stock, $0.001 par value	Common stock, $0.001 par value Effect of the Adoption of ASU 2018-12	Additional paid-in capital	Additional paid-in capital Effect of the Adoption of ASU 2018-12	Retained earnings	Retained earnings Effect of the Adoption of ASU 2018-12		Accumulated other comprehensive income (loss), net of tax:	Accumulated other comprehensive income (loss), net of tax: Effect of the Adoption of ASU 2018-12	Treasury stock, at cost	Treasury stock, at cost Effect of the Adoption of ASU 2018-12
Accounting Standards Update [Extensible List]	Accounting Standards Update 2018-12
Ending balance at Dec. 31, 2021			$ 0.1		$ 495.3		$ 1,547.0	$ 22.1		$ (50.0)	$ (330.5)	$ (493.4)
Ending balance (Accounting Standards Update 2018-12) at Dec. 31, 2021							(0.8)	(0.8)	[1]
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units					(1.2)
Share-based compensation expense					4.0
Net income (loss)	$ 16.1	$ 14.1					16.1	14.1
Dividends, 2023, $0.33 per share; 2022, $0.32 per share							(27.0)	0.0
Change in net unrealized investment losses on fixed maturity securities										(628.9)	(117.8)
Change in net reserve remeasurements attributable to discount rates	350.0	350.0								350.0	350.0
Treasury stock acquired - share repurchase authorization												(14.0)
Ending balance at Jun. 30, 2022	1,197.2	(62.9)	0.1	$ 0.0	498.1	$ 0.0	1,535.3	35.4		(328.9)	(98.3)	(507.4)	$ 0.0
Beginning balance at Mar. 31, 2022			0.1		496.6		1,553.0	27.1		(202.6)	(212.4)	(495.6)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units					(0.7)
Share-based compensation expense					2.2
Net income (loss)	(4.2)	8.3					(4.2)	8.3
Dividends, 2023, $0.33 per share; 2022, $0.32 per share							(13.5)	0.0
Change in net unrealized investment losses on fixed maturity securities										(294.8)	(54.4)
Change in net reserve remeasurements attributable to discount rates	168.5	168.5								168.5	168.5
Treasury stock acquired - share repurchase authorization												(11.8)
Ending balance at Jun. 30, 2022	1,197.2	(62.9)	0.1	$ 0.0	498.1	$ 0.0	1,535.3	$ 35.4		(328.9)	$ (98.3)	(507.4)	$ 0.0
Beginning balance at Dec. 31, 2022	1,098.3	$ 10.1	0.1		502.6		1,512.4			(399.4)		(517.4)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units					(1.1)
Share-based compensation expense					4.2
Net income (loss)	(6.2)						(6.2)
Dividends, 2023, $0.33 per share; 2022, $0.32 per share							(27.6)
Change in net unrealized investment losses on fixed maturity securities										55.9
Change in net reserve remeasurements attributable to discount rates	(16.1)									(16.1)
Treasury stock acquired - share repurchase authorization												(5.5)
Ending balance at Jun. 30, 2023	1,101.9		0.1		505.7		1,478.6			(359.6)		(522.9)
Beginning balance at Mar. 31, 2023			0.1		503.1		1,505.2			(347.4)		(521.8)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units					0.3
Share-based compensation expense					2.3
Net income (loss)	(12.8)						(12.8)
Dividends, 2023, $0.33 per share; 2022, $0.32 per share							(13.8)
Change in net unrealized investment losses on fixed maturity securities										(37.3)
Change in net reserve remeasurements attributable to discount rates	25.1									25.1
Treasury stock acquired - share repurchase authorization												(1.1)
Ending balance at Jun. 30, 2023	$ 1,101.9		$ 0.1		$ 505.7		$ 1,478.6			$ (359.6)		$ (522.9)

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