Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Long Duration Insurance Contracts [Member]	Retained Earnings [Member]	Retained Earnings [Member] Adjustment [Member]	Retained Earnings [Member] Adjusted Balance [Member]	Accumulated Other Comprehensive Income (Loss), Net of Tax [Member]	Treasury Stock [Member]
Balance at beginning of period at Dec. 31, 2023		$ 0.6	$ 1,939.2	$ (462.8)	$ (54.8)	$ 0.4	$ 2,909.4		$ 2,909.4		$ (1,866.4)
Net income	$ 115.5						115.5
Employee and director stock-based awards and other			2.1
Net appreciation (depreciation) on available-for-sale securities	(34.1)			(34.1)
Net amount recognized as net periodic benefit cost					1.4
Net change in market risk						1.0
Dividends to shareholders							(31.0)
Net shares reissued at cost under employee stock-based compensation plans											2.2
Balance at end of period at Mar. 31, 2024	2,522.7		1,941.3	(496.9)	(53.4)	1.4	2,993.9			$ (548.9)	(1,864.2)
Balance at beginning of period at Dec. 31, 2024	2,841.8	$ 0.6	1,973.6	(403.5)	(55.2)	2.4	3,209.6	$ 1.9	$ 3,211.5		(1,885.7)
Balance at beginning of period at Dec. 31, 2024	(1,885.7)
Net income	128.2						128.2
Shares purchased at cost											(11.0)
Employee and director stock-based awards and other			4.7
Net appreciation (depreciation) on available-for-sale securities	111.2			111.2
Net amount recognized as net periodic benefit cost					1.4
Net change in market risk						(1.0)
Dividends to shareholders							(32.9)
Net shares reissued at cost under employee stock-based compensation plans											0.1
Balance at end of period at Mar. 31, 2025	$ 3,044.4		$ 1,978.3	$ (292.3)	$ (53.8)	$ 1.4	$ 3,306.8			$ (344.7)	$ (1,896.6)