Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Net Unrealized Appreciation 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 [Member]
Balance at beginning of period at Dec. 31, 2015		$ 0.6	$ 1,833.5	$ 149.9	$ (78.6)	$ (17.4)		$ 1,803.5	$ (847.1)
Net income	$ 168.6							168.6
Shares purchased at cost									(105.2)
Employee and director stock-based awards and other			5.4
Net appreciation on available-for-sale securities	171.5			171.5
Net amount recognized as net periodic benefit cost					4.8
Amount recognized as cumulative foreign currency translation during the period	(3.2)					(3.2)
Dividends to shareholders								(59.2)
Net shares reissued at cost under employee stock-based compensation plans									21.1
Stock-based compensation	(8.5)							(2.5)
Balance at end of period at Sep. 30, 2016	3,045.7	0.6	1,838.9	321.4	(73.8)	(20.6)	$ 227.0	1,910.4	(931.2)
Balance at beginning of period at Dec. 31, 2016	2,857.5	0.6	1,846.7	186.0	(102.5)	(20.7)		1,875.6	(928.2)
Net income	134.7							134.7
Shares purchased at cost	(37.2)								(37.2)
Employee and director stock-based awards and other			9.7
Net appreciation on available-for-sale securities	42.5			42.5
Net amount recognized as net periodic benefit cost					7.0
Amount recognized as cumulative foreign currency translation during the period	1.7					1.7
Dividends to shareholders								(63.8)
Net shares reissued at cost under employee stock-based compensation plans									19.9
Stock-based compensation	(11.5)
Balance at end of period at Sep. 30, 2017	$ 2,972.0	$ 0.6	$ 1,856.4	$ 228.5	$ (95.5)	$ (19.0)	$ 114.0	$ 1,946.5	$ (945.5)