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]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss), Net of Tax [Member]	Treasury Stock [Member]
Balance at beginning of period at Dec. 31, 2020		$ 0.6	$ 1,857.4	$ 428.1	$ (55.6)	$ 2,668.0
Balance at beginning of period at Dec. 31, 2020								$ (1,696.3)
Net income	$ 221.2					221.2
Shares purchased at cost								(123.6)
Employee and director stock-based awards and other			16.0
Net appreciation (depreciation) on available-for-sale securities	(123.4)			(123.4)
Net amount recognized as net periodic benefit					1.4
Dividends to shareholders						(51.3)
Net shares reissued at cost under employee stock-based compensation plans								11.5
Balance at end of period at Jun. 30, 2021	3,154.0		1,873.4	304.7	(54.2)	2,837.9	$ 250.5
Balance at end of period at Jun. 30, 2021								(1,808.4)
Balance at beginning of period at Mar. 31, 2021		0.6	1,864.0	242.6	(54.9)	2,734.9
Balance at beginning of period at Mar. 31, 2021								(1,740.4)
Net income	128.5					128.5
Shares purchased at cost								(73.3)
Employee and director stock-based awards and other			9.4
Net appreciation (depreciation) on available-for-sale securities	62.1			62.1
Net amount recognized as net periodic benefit					0.7
Dividends to shareholders						(25.5)
Net shares reissued at cost under employee stock-based compensation plans								5.3
Balance at end of period at Jun. 30, 2021	3,154.0		1,873.4	304.7	(54.2)	2,837.9	250.5
Balance at end of period at Jun. 30, 2021								(1,808.4)
Balance at beginning of period at Dec. 31, 2021	3,144.9	0.6	1,887.2	184.9	(62.7)	2,983.2
Balance at beginning of period at Dec. 31, 2021	(1,848.3)							(1,848.3)
Net income	127.4					127.4
Shares purchased at cost								(20.4)
Employee and director stock-based awards and other			9.6
Net appreciation (depreciation) on available-for-sale securities	(644.3)			(644.3)
Net amount recognized as net periodic benefit					2.2
Dividends to shareholders						(53.9)
Net shares reissued at cost under employee stock-based compensation plans								6.3
Balance at end of period at Jun. 30, 2022	2,571.8		1,896.8	(459.4)	(60.5)	3,056.7	(519.9)
Balance at end of period at Jun. 30, 2022	(1,862.4)							(1,862.4)
Balance at beginning of period at Mar. 31, 2022		$ 0.6	1,887.8	(195.0)	(61.6)	3,061.0
Balance at beginning of period at Mar. 31, 2022								(1,860.0)
Net income	22.6					22.6
Shares purchased at cost								(4.1)
Employee and director stock-based awards and other			9.0
Net appreciation (depreciation) on available-for-sale securities	(264.4)			(264.4)
Net amount recognized as net periodic benefit					1.1
Dividends to shareholders						(26.9)
Net shares reissued at cost under employee stock-based compensation plans								1.7
Balance at end of period at Jun. 30, 2022	2,571.8		$ 1,896.8	$ (459.4)	$ (60.5)	$ 3,056.7	$ (519.9)
Balance at end of period at Jun. 30, 2022	$ (1,862.4)							$ (1,862.4)