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)	$ 128.1						128.1
Shares purchased at cost											(30.8)
Employee and director stock-based awards and other			17.3
Net depreciation on available-for-sale securities	(891.6)			(891.6)
Net amount recognized as net periodic benefit					3.3
Net change in market risk						17.4
Dividends to shareholders							(81.2)
Net shares reissued at cost under employee stock-based compensation plans											6.4
Balance at end of period at Sep. 30, 2022	2,304.9		1,904.5	(706.7)	(59.4)	4.4	3,034.2			$ (761.7)
Balance at end of period at Sep. 30, 2022											(1,872.7)
Balance at beginning of period at Jun. 30, 2022		0.6	1,896.8	(459.4)	(60.5)	(0.1)	3,061.0		3,061.0
Balance at beginning of period at Jun. 30, 2022											(1,862.4)
Net income (loss)	0.5						0.5
Shares purchased at cost											(10.4)
Employee and director stock-based awards and other			7.7
Net depreciation on available-for-sale securities	(247.3)			(247.3)
Net amount recognized as net periodic benefit					1.1
Net change in market risk						4.3
Dividends to shareholders							(27.3)
Net shares reissued at cost under employee stock-based compensation plans											0.1
Balance at end of period at Sep. 30, 2022	2,304.9		1,904.5	(706.7)	(59.4)	4.4	3,034.2			(761.7)
Balance at end of period at Sep. 30, 2022											(1,872.7)
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)	(72.6)						$ (72.6)
Shares purchased at cost											0.0
Employee and director stock-based awards and other			18.3
Net depreciation on available-for-sale securities	$ (86.1)			86.1
Net amount recognized as net periodic benefit					4.7
Accounting Standards Update [Extensible Enumeration]	ASU 2018-12					ASU 2018-12	ASU 2018-12
Net change in market risk						$ (2.6)
Dividends to shareholders							$ (88.0)
Net shares reissued at cost under employee stock-based compensation plans											3.7
Balance at end of period at Sep. 30, 2023	$ 2,116.3		1,931.4	(727.5)	(59.4)	6.6	2,832.3			(780.3)
Balance at end of period at Sep. 30, 2023	(1,867.7)										(1,867.7)
Balance at beginning of period at Jun. 30, 2023		$ 0.6	1,923.0	(613.3)	(60.9)	3.3	2,853.0		$ 2,853.0
Balance at beginning of period at Jun. 30, 2023											(1,867.8)
Net income (loss)	8.6						8.6
Employee and director stock-based awards and other			8.4
Net depreciation on available-for-sale securities	(114.2)			(114.2)
Net amount recognized as net periodic benefit					1.5
Net change in market risk						3.3
Dividends to shareholders							(29.3)
Net shares reissued at cost under employee stock-based compensation plans											0.1
Balance at end of period at Sep. 30, 2023	2,116.3		$ 1,931.4	$ (727.5)	$ (59.4)	$ 6.6	$ 2,832.3			$ (780.3)
Balance at end of period at Sep. 30, 2023	$ (1,867.7)										$ (1,867.7)