Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member]	Net Unrealized Appreciation (Depreciation) on Investments [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Defined Benefit Pension and Postretirement Plans [Member]	Defined Benefit Pension and Postretirement Plans [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Cumulative Foreign Currency Translation Adjustment [Member]	Cumulative Foreign Currency Translation Adjustment [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment [Member]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance [Member]	Treasury Stock	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning year at Dec. 31, 2017		$ 0.6	$ 1,857.0	$ 205.4		$ (79.5)		$ (18.3)		$ 1,975.0	$ 104.3	$ 2,079.3
Balance at beginning year (Accounting Standards Updates No. 2016-01 and No. 2018-02) at Dec. 31, 2017					$ (81.6)		$ (16.2)		$ (3.9)
Balance at beginning of year at Dec. 31, 2017													$ (942.5)
Net income	$ 391.0									391.0
Shares purchased at cost													(57.7)
Employee and director stock-based awards and other			14.8
Net appreciation (depreciation) on available-for-sale securities	(151.0)			(170.1)
Net amount arising in the period	(17.0)					(17.0)
Net amount recognized as net periodic benefit cost	7.7					7.7
Amount recognized as cumulative foreign currency translation during the year	21.5							(1.7)
Amount realized with sale of Chaucer business				19.1		16.4		23.2
Dividends to shareholders										(288.0)
Net shares reissued at cost under employee stock-based compensation plans													16.7
Balance at end of year at Dec. 31, 2018	2,954.7	0.6	1,871.8	(27.2)	$ 1.5	(88.6)		(0.7)		2,182.3	(1.5)	2,180.8		$ (116.5)
Balance at end of year at Dec. 31, 2018													(983.5)
Net income	425.1									425.1
Settlement and prepayment of accelerated share repurchases			(49.5)
Shares purchased at cost													(514.1)
Employee and director stock-based awards and other			15.0
Net appreciation (depreciation) on available-for-sale securities	241.7			$ 241.6
Net amount arising in the period	16.2					16.2
Net amount recognized as net periodic benefit cost	9.0					9.0
Amount recognized as cumulative foreign currency translation during the year	0.7
Accounting Standards Update Extensible List				us-gaap:AccountingStandardsUpdate201708Member
Amount realized with sale of Chaucer business				$ 0.1				$ 0.7
Dividends to shareholders										(195.0)
Net shares reissued at cost under employee stock-based compensation plans													12.4
Balance at end of year at Dec. 31, 2019	2,916.2	0.6	1,837.3	216.0		(63.4)				2,410.9	$ (0.9)	$ 2,410.0		152.6
Balance at end of year at Dec. 31, 2019	(1,485.2)												(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				$ 372.5
Balance at end of year at Dec. 31, 2020	$ (1,696.3)												$ (1,696.3)