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, 2021		$ 0.6	$ 1,887.2	$ 184.9	$ (62.7)		$ 2,983.2		$ 2,987.3
Balance at beginning of period (ASU 2018-12) at Dec. 31, 2021						$ (13.0)		$ 4.1
Balance at beginning of period at Dec. 31, 2021											$ (1,848.3)
Net income (loss)	$ 104.9						104.9
Shares purchased at cost											(16.3)
Employee and director stock-based awards and other			0.6
Net appreciation (depreciation) on available-for-sale securities	(379.9)			(379.9)
Net amount recognized as net periodic benefit					1.1
Net change in market risk						6.9
Dividends to shareholders							(27.0)
Net shares reissued at cost under employee stock-based compensation plans											4.6
Balance at end of period at Mar. 31, 2022	2,830.9		1,887.8	(195.0)	(61.6)	(6.1)	3,065.2			$ (262.7)
Balance at end of period at Mar. 31, 2022											(1,860.0)
Balance at beginning of period at Dec. 31, 2022	2,333.7	$ 0.6	1,913.1	(641.4)	(64.1)	$ 4.0	2,992.9		$ 2,992.9
Balance at beginning of period at Dec. 31, 2022	(1,871.4)										(1,871.4)
Net income (loss)	(12.0)						$ (12.0)
Employee and director stock-based awards and other			0.5
Net appreciation (depreciation) on available-for-sale securities	$ 94.3			94.3
Net amount recognized as net periodic benefit					1.6
Accounting Standards Update [Extensible Enumeration]	ASU 2018-12					ASU 2018-12	ASU 2018-12
Net change in market risk						$ (2.1)
Dividends to shareholders							$ (29.3)
Net shares reissued at cost under employee stock-based compensation plans											2.3
Balance at end of period at Mar. 31, 2023	$ 2,389.0		$ 1,913.6	$ (547.1)	$ (62.5)	$ 1.9	$ 2,951.6			$ (607.7)
Balance at end of period at Mar. 31, 2023	$ (1,869.1)										$ (1,869.1)