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	$ 255.2					255.2
Shares purchased at cost								(147.6)
Employee and director stock-based awards and other			23.3
Net depreciation on available-for-sale securities	(171.3)			(171.3)
Net amount recognized as net periodic benefit					2.0
Dividends to shareholders						(76.5)
Net shares reissued at cost under employee stock-based compensation plans								15.0
Balance at end of period at Sep. 30, 2021	3,102.3		1,880.7	256.8	(53.6)	2,846.7	$ 203.2
Balance at end of period at Sep. 30, 2021								(1,828.9)
Balance at beginning of period at Jun. 30, 2021		0.6	1,873.4	304.7	(54.2)	2,837.9
Balance at beginning of period at Jun. 30, 2021								(1,808.4)
Net income	34.0					34.0
Shares purchased at cost								(24.0)
Employee and director stock-based awards and other			7.3
Net depreciation on available-for-sale securities	(47.9)			(47.9)
Net amount recognized as net periodic benefit					0.6
Dividends to shareholders						(25.2)
Net shares reissued at cost under employee stock-based compensation plans								3.5
Balance at end of period at Sep. 30, 2021	3,102.3		1,880.7	256.8	(53.6)	2,846.7	203.2
Balance at end of period at Sep. 30, 2021								(1,828.9)
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	127.6					127.6
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
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	2,295.9		1,904.5	(706.7)	(59.4)	3,029.6	(766.1)
Balance at end of period at Sep. 30, 2022	(1,872.7)							(1,872.7)
Balance at beginning of period at Jun. 30, 2022		$ 0.6	1,896.8	(459.4)	(60.5)	3,056.7
Balance at beginning of period at Jun. 30, 2022								(1,862.4)
Net income	0.2					0.2
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
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	2,295.9		$ 1,904.5	$ (706.7)	$ (59.4)	$ 3,029.6	$ (766.1)
Balance at end of period at Sep. 30, 2022	$ (1,872.7)							$ (1,872.7)