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]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Income (Loss), net of tax [Member]
Balance at beginning of period at Dec. 31, 2016		$ 0.6	$ 1,846.7	$ 186.0	$ (102.5)	$ (20.7)	$ 1,875.6	$ (928.2)
Balance at beginning of period, as adjusted at Dec. 31, 2016							1,875.6
Net income	$ 134.7						134.7
Shares purchased at cost								(37.2)
Employee and director stock-based awards and other			9.7
Net appreciation (depreciation) on available-for-sale securities	42.5			42.5
Net change in net actuarial loss and prior service cost	7.0				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
Balance at end of period at Sep. 30, 2017	2,972.0	0.6	1,856.4	228.5	(95.5)	(19.0)	1,946.5	(945.5)	$ 114.0
Balance at beginning of period at Dec. 31, 2017	2,997.7	0.6	1,857.0	205.4	(79.5)	(18.3)	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	267.4						267.4
Shares purchased at cost	(44.2)							(44.2)
Employee and director stock-based awards and other			11.9
Net appreciation (depreciation) on available-for-sale securities	(197.8)			(197.8)
Net change in net actuarial loss and prior service cost	0.5				0.5
Amount recognized as cumulative foreign currency translation during the period	0.6					0.6
Adoption of Accounting Standards Update | Accounting Standards Update 2016-01 and 2018-02 [Member]				(81.6)
Adoption of Accounting Standards Update | Accounting Standards Update 2018-02 [Member]					(16.2)	(3.9)
Dividends to shareholders							(69.1)
Net shares reissued at cost under employee stock-based compensation plans								12.8
Balance at end of period at Sep. 30, 2018	$ 2,982.4	$ 0.6	$ 1,868.9	$ (74.0)	$ (95.2)	$ (21.6)	$ 2,277.6	$ (973.9)	$ (190.8)