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 of period at Dec. 31, 2023		$ 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			(0.1)
Share-based compensation expense			2.3
Net income	$ 26.5			26.5
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(14.3)
Change in net unrealized investment losses on fixed maturity securities					(19.7)
Change in net reserve remeasurements attributable to discount rates	41.4				41.4
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						0.0
End of period at Mar. 31, 2024	1,211.4	0.1	513.1	1,514.4	(292.3)	(523.9)
Beginning of period at Dec. 31, 2024	1,287.5	0.1	525.2	1,548.2	(253.5)	(532.5)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units			(1.6)
Share-based compensation expense			2.5
Net income	38.2			38.2
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(14.7)
Change in net unrealized investment losses on fixed maturity securities					42.9
Change in net reserve remeasurements attributable to discount rates	(11.8)				(11.8)
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(0.2)
End of period at Mar. 31, 2025	$ 1,342.8	$ 0.1	$ 526.1	$ 1,571.7	$ (222.4)	$ (532.7)