CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - 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 [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			(0.5)
Share-based compensation expense			8.8
Net income	$ 45.0			45.0
Dividends, 2025, $1.40 per share; 2024, $1.36 per share; 2023, $1.32 per share				(55.2)
Change in net unrealized investment gains (losses) on fixed maturity securities					121.3
Change in net reserve remeasurements attributable to discount rates	(37.1)				(37.1)
Change in net funded status of benefit plans					1.2
Treasury stock acquired - share repurchase authorization						(6.5)
Ending balance at Dec. 31, 2023	1,175.3	0.1	510.9	1,502.2	(314.0)	(523.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			4.9
Share-based compensation expense			9.4
Net income	102.8			102.8
Dividends, 2025, $1.40 per share; 2024, $1.36 per share; 2023, $1.32 per share				(56.8)
Change in net unrealized investment gains (losses) on fixed maturity securities					(29.1)
Change in net reserve remeasurements attributable to discount rates	89.0				89.0
Change in net funded status of benefit plans					0.6
Treasury stock acquired - share repurchase authorization						(8.6)
Ending balance at Dec. 31, 2024	1,287.5	0.1	525.2	1,548.2	(253.5)	(532.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			3.4
Share-based compensation expense			10.2
Net income	162.1			162.1
Dividends, 2025, $1.40 per share; 2024, $1.36 per share; 2023, $1.32 per share				(58.6)
Change in net unrealized investment gains (losses) on fixed maturity securities					111.9
Change in net reserve remeasurements attributable to discount rates	(17.5)				(17.5)
Change in net funded status of benefit plans					4.5
Treasury stock acquired - share repurchase authorization						(20.8)
Ending balance at Dec. 31, 2025	$ 1,482.7	$ 0.1	$ 538.8	$ 1,651.7	$ (154.6)	$ (553.3)