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]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning year at Dec. 31, 2019		$ 0.6	$ 1,837.3	$ 216.0	$ (63.4)	$ 2,410.9	$ (0.9)	$ 2,410.0
Balance at beginning of year at Dec. 31, 2019									$ (1,485.2)
Net income	$ 358.7					358.7
Settlement and prepayment of accelerated share repurchases			4.4
Shares purchased at cost									(217.2)
Employee and director stock-based awards and other			15.7
Net appreciation (depreciation) on available-for-sale securities	212.1			212.1
Net amount arising in the period	3.1				3.1
Net amount recognized as net periodic benefit cost	4.7				4.7
Dividends to shareholders						(100.7)
Net shares reissued at cost under employee stock-based compensation plans									6.1
Balance at end of year at Dec. 31, 2020	3,202.2	0.6	1,857.4	428.1	(55.6)	2,668.0		2,668.0		$ 372.5
Balance at end of year at Dec. 31, 2020									(1,696.3)
Net income	418.7					418.7
Settlement and prepayment 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
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	3,144.9	0.6	1,887.2	184.9	(62.7)	2,983.2		$ 2,983.2		122.2
Balance at end of year at Dec. 31, 2021	(1,848.3)								(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
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	2,325.6	$ 0.6	$ 1,913.1	$ (641.4)	$ (64.1)	$ 2,988.8				$ (705.5)
Balance at end of year at Dec. 31, 2022	$ (1,871.4)								$ (1,871.4)