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	$ 45.2						45.2
Shares purchased at cost								(4.6)
Employee and director stock-based awards and other			0.5
Net (depreciation) appreciation on available-for-sale securities	19.0			19.0
Net change in net actuarial loss and prior service cost	2.3				2.3
Amount recognized as cumulative foreign currency translation during the period	5.1					5.1
Dividends to shareholders							(21.4)
Net shares reissued at cost under employee stock-based compensation plans								9.9
Stock-based compensation	(3.2)
Balance at end of period at Mar. 31, 2017	2,913.5	0.6	1,847.2	205.0	(100.2)	(15.6)	1,899.4	(922.9)	$ 89.2
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	67.7						67.7
Shares purchased at cost	(13.6)							(13.6)
Employee and director stock-based awards and other			1.3
Net (depreciation) appreciation on available-for-sale securities	(205.1)			(123.5)
Net change in net actuarial loss and prior service cost	(2.3)				(2.3)
Amount recognized as cumulative foreign currency translation during the period	(0.5)					(0.5)
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							(22.9)
Net shares reissued at cost under employee stock-based compensation plans								6.6
Stock-based compensation	(3.0)
Balance at end of period at Mar. 31, 2018	$ 2,913.1	$ 0.6	$ 1,858.3	$ 0.3	$ (98.0)	$ (22.7)	$ 2,124.1	$ (949.5)	$ (120.4)