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]	Cumulative Foreign Currency Translation Adjustment [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]	Retained Earnings [Member]	Treasury Stock
Balance at beginning year at Dec. 31, 2015		$ 0.6	$ 1,833.5	$ 149.9	$ (78.6)	$ (17.4)		$ 1,803.5	$ (847.1)
Balance at beginning of period, as adjusted at Dec. 31, 2015								1,803.5
Net income	$ 155.1							155.1
Shares purchased at cost									(105.6)
Employee and director stock-based awards and other			13.2
Net appreciation (depreciation) on available-for-sale securities	36.1			36.1
Net amount arising in the period	(30.3)				(30.3)
Net amount recognized as net periodic benefit cost	6.4				6.4
Amount recognized as cumulative foreign currency translation during the period	(3.3)					(3.3)
Dividends to shareholders								(80.4)
Net shares reissued at cost under employee stock-based compensation plans									24.5
Stock-based compensation								(2.6)
Balance at end of year at Dec. 31, 2016	2,857.5	0.6	1,846.7	186.0	(102.5)	(20.7)	$ 62.8	1,875.6	(928.2)
Balance at beginning of period, as adjusted at Dec. 31, 2016								1,875.6
Net income	186.2							186.2
Shares purchased at cost									(37.2)
Employee and director stock-based awards and other			10.3
Net appreciation (depreciation) on available-for-sale securities	19.4			19.4
Net amount arising in the period	13.6				13.6
Net amount recognized as net periodic benefit cost	9.4				9.4
Amount recognized as cumulative foreign currency translation during the period	2.4					2.4
Dividends to shareholders								(86.8)
Net shares reissued at cost under employee stock-based compensation plans									22.9
Balance at end of year at Dec. 31, 2017	2,997.7	$ 0.6	1,857.0	205.4	(79.5)	(18.3)	107.6	1,975.0	(942.5)
Cumulative effect of accounting change, net of taxes at Dec. 31, 2017								104.3
Balance at beginning of period, as adjusted at Dec. 31, 2017								2,079.3
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 period	21.5					(1.7)
Adoption of Accounting Standards Update | Accounting Standards Update 2018-02 [Member]					(16.2)	(3.9)
Adoption of Accounting Standards Update | Accounting Standards Update 2016-01 and 2018-02 [Member]				(81.6)
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		$ 1,871.8	$ (27.2)	$ (88.6)	$ (0.7)	$ (116.5)	$ 2,182.3	$ 983.5