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]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning year at Dec. 31, 2020		$ 0.6	$ 1,857.4	$ 428.1	$ (55.6)		$ 2,668.0
Balance at beginning year (ASU 2018-12) at Dec. 31, 2020						$ (17.2)
Balance at beginning of year at Dec. 31, 2020								$ (1,696.3)
Net income	$ 422.8						422.8
Settlement of accelerated share repurchases			5.0
Shares purchased at cost								(167.6)
Employee and director stock-based awards and other			24.8
Net appreciation (depreciation) on available-for-sale securities	(243.2)			(243.2)
Net amount arising in the period	(9.8)				(9.8)
Net amount recognized as net periodic benefit cost	2.7				2.7
Net change in market risk						4.2
Dividends to shareholders							(103.5)
Net shares reissued at cost under employee stock-based compensation plans								15.6
Balance at end of year at Dec. 31, 2021	3,136.0	0.6	1,887.2	184.9	(62.7)	(13.0)	2,987.3		$ 109.2
Balance at end of year at Dec. 31, 2021								(1,848.3)
Net income	116.0						116.0
Shares purchased at cost								(30.8)
Employee and director stock-based awards and other			25.9
Net appreciation (depreciation) on available-for-sale securities	(826.3)			(826.3)
Net amount arising in the period	(5.7)				(5.7)
Net amount recognized as net periodic benefit cost	4.3				4.3
Net change in market risk						17.0
Dividends to shareholders							(110.4)
Net shares reissued at cost under employee stock-based compensation plans								7.7
Balance at end of year at Dec. 31, 2022	2,333.7	$ 0.6	1,913.1	(641.4)	(64.1)	$ 4.0	2,992.9		(701.5)
Balance at end of year at Dec. 31, 2022	(1,871.4)							(1,871.4)
Net income	35.3						35.3
Employee and director stock-based awards and other			26.1
Net appreciation (depreciation) on available-for-sale securities	178.6			178.6
Net amount arising in the period	3.1				(3.1)
Net amount recognized as net periodic benefit cost	$ 6.2				6.2
Accounting Standards Update [Extensible Enumeration]	ASU 2018-12					ASU 2018-12
Net change in market risk						$ (3.6)
Dividends to shareholders							(118.8)
Net shares reissued at cost under employee stock-based compensation plans								5.0
Balance at end of year at Dec. 31, 2023	$ 2,465.6		$ 1,939.2	$ (462.8)	$ (54.8)	$ 0.4	$ 2,909.4		$ (517.2)
Balance at end of year at Dec. 31, 2023	$ (1,866.4)							$ (1,866.4)