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			0.5
Share-based compensation expense			6.9
Net income	$ 64.6			64.6
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(42.7)
Change in net unrealized investment losses on fixed maturity securities					91.1
Change in net reserve remeasurements attributable to discount rates	(0.1)				(0.1)
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(8.6)
End of period at Sep. 30, 2024	1,287.0	0.1	518.3	1,524.1	(223.0)	(532.5)
Beginning of period at Dec. 31, 2023		0.1	510.9	1,502.2	(314.0)	(523.9)
End of period at Dec. 31, 2024	1,287.5	0.1	525.2	1,548.2	(253.5)	(532.5)
Beginning of period at Jun. 30, 2024		0.1	515.8	1,504.0	(282.7)	(528.4)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units			0.3
Share-based compensation expense			2.2
Net income	34.3			34.3
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(14.2)
Change in net unrealized investment losses on fixed maturity securities					133.2
Change in net reserve remeasurements attributable to discount rates	(73.5)				(73.5)
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(4.1)
End of period at Sep. 30, 2024	1,287.0	0.1	518.3	1,524.1	(223.0)	(532.5)
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			3.1
Share-based compensation expense			7.5
Net income	125.9			125.9
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(43.9)
Change in net unrealized investment losses on fixed maturity securities					106.6
Change in net reserve remeasurements attributable to discount rates	(31.5)				(31.5)
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(13.9)
End of period at Sep. 30, 2025	1,441.3	0.1	535.8	1,630.2	(178.4)	(546.4)
Beginning of period at Jun. 30, 2025		0.1	530.4	1,586.5	(217.1)	(539.6)
Increase (Decrease) in Stockholders' Equity
Options exercised and conversion of common and restricted stock units			3.0
Share-based compensation expense			2.4
Net income	58.3			58.3
Dividends, 2025, $0.35 per share; 2024, $0.34 per share				(14.6)
Change in net unrealized investment losses on fixed maturity securities					59.1
Change in net reserve remeasurements attributable to discount rates	(20.4)				(20.4)
Change in net funded status of benefit plans					0.0
Treasury stock acquired - share repurchase authorization						(6.8)
End of period at Sep. 30, 2025	$ 1,441.3	$ 0.1	$ 535.8	$ 1,630.2	$ (178.4)	$ (546.4)