CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of tax:	Treasury stock, at cost
Beginning balance at Dec. 31, 2022		$ 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, 2024, $0.34 per share; 2023, $0.33 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)
Change in net funded status of benefit plans					0.0
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, 2024, $0.34 per share; 2023, $0.33 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
Change in net funded status of benefit plans					0.0
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)
Beginning balance at Dec. 31, 2023	1,175.3	0.1	510.9	1,502.2	(314.0)	(523.9)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units			0.2
Share-based compensation expense			4.7
Net income (loss)	30.3			30.3
Dividends, 2024, $0.34 per share; 2023, $0.33 per share				(28.5)
Change in net unrealized investment losses on fixed maturity securities					(42.1)
Change in net reserve remeasurements attributable to discount rates	73.4				73.4
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(4.5)
Ending balance at Jun. 30, 2024	1,208.8	0.1	515.8	1,504.0	(282.7)	(528.4)
Beginning balance at Mar. 31, 2024		0.1	513.1	1,514.4	(292.3)	(523.9)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units			0.3
Share-based compensation expense			2.4
Net income (loss)	3.8			3.8
Dividends, 2024, $0.34 per share; 2023, $0.33 per share				(14.2)
Change in net unrealized investment losses on fixed maturity securities					(22.4)
Change in net reserve remeasurements attributable to discount rates	32.0				32.0
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(4.5)
Ending balance at Jun. 30, 2024	$ 1,208.8	$ 0.1	$ 515.8	$ 1,504.0	$ (282.7)	$ (528.4)