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, 2017		$ 0.6	$ 1,857.0	$ 205.4	$ (79.5)	$ (18.3)	$ 1,975.0
Balance at beginning of period at Dec. 31, 2017								$ (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)
Prepaid repurchase of common stock and other			1.3
Net appreciation (depreciation) on available-for-sale securities	(123.5)			(123.5)
Net amount recognized as net periodic benefit (expense)	(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 No. 2017-08 and No. 2016-01 and No. 2018-02 [Member]				(81.6)
Adoption of Accounting Standards Update | Accounting Standards Update No. 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
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		$ (120.4)
Balance at end of period at Mar. 31, 2018								(949.5)
Balance at beginning of period at Dec. 31, 2018	2,954.7	0.6	1,871.8	(27.2)	(88.6)	(0.7)	2,182.3
Balance at beginning of period at Dec. 31, 2018	(983.5)							(983.5)
Cumulative effect of accounting change, net of taxes at Dec. 31, 2018							(1.5)
Balance at beginning of period, as adjusted at Dec. 31, 2018							2,180.8
Net income	122.4						122.4
Shares purchased at cost								(200.0)
Prepaid repurchase of common stock and other			(50.1)
Net appreciation (depreciation) on available-for-sale securities	116.4			116.4
Net amount recognized as net periodic benefit (expense)	2.3				2.3
Amount recognized as cumulative foreign currency translation during the period	0.1					0.1
Adoption of Accounting Standards Update | Accounting Standards Update No. 2017-08 and No. 2016-01 and No. 2018-02 [Member]				1.5
Dividends to shareholders							(24.5)
Net shares reissued at cost under employee stock-based compensation plans								5.7
Balance at end of period at Mar. 31, 2019	2,927.0	$ 0.6	$ 1,821.7	$ 90.7	$ (86.3)	$ (0.6)	$ 2,278.7		$ 3.8
Balance at end of period at Mar. 31, 2019	$ (1,177.8)							$ (1,177.8)