CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Millions		Total		Common stock		Additional paid-in capital		Retained earnings		Retained earnings Effect of adopting ASU 2018-12	Accumulated other comprehensive income (loss), net of tax:		Treasury stock, at cost
Beginning balance at Dec. 31, 2021	[1]			$ 0.1		$ 495.3		$ 1,547.0		$ (0.7)	$ (50.0)		$ (493.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units	[1]					(0.9)
Share-based compensation expense	[1]					8.2
Net income		$ 19.8	[2]					19.8	[1]
Dividends, 2024, $1.36 per share; 2023, $1.32 per share; 2022, $1.28 per share	[1]							(53.7)
Change in net unrealized investment gains (losses) on fixed maturity securities		(796.7)	[2]								(796.7)	[1]
Change in net reserve remeasurements attributable to discount rates		445.9	[2]								445.9	[1]
Change in net funded status of benefit plans		1.4	[2]								1.4	[1]
Treasury stock acquired - share repurchase authorization	[1]												(24.0)
Ending balance at Dec. 31, 2022		1,098.3	[1]	0.1	[1]	502.6	[1]	1,512.4	[1]	0.0	(399.4)	[1]	(517.4)	[1]
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, 2024, $1.36 per share; 2023, $1.32 per share; 2022, $1.28 per share								(55.2)
Change in net unrealized investment gains (losses) on fixed maturity securities		121.3									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									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		$ 0.0	(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, 2024, $1.36 per share; 2023, $1.32 per share; 2022, $1.28 per share								(56.8)
Change in net unrealized investment gains (losses) on fixed maturity securities		(29.1)									(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									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)

[1] Recast for the adoption of ASU 2018-12.

[2] Recast for the adoption of ASU 2018-12.