Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Preferred Stock [Member]	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss), net of tax	Net Unrealized Appreciation on Investments and Derivative Instruments:	Defined Benefit Pension and Postretirement Plans:	Cumulative Foreign Currency Translation Adjustment:	Retained Earnings	Treasury Stock	Total
Balance at beginning of period at Dec. 31, 2013			$ 1,830.1		$ 259.3	$ (76.1)	$ (5.6)	$ 1,349.1	$ (762.9)
Net income								192.1		$ 192.1
Shares purchased at cost									(20.4)
Employee and director stock-based awards and other			(2.7)
Net (depreciation) appreciation on available-for-sale securities and derivative instruments					34.4					34.4
Net amount recognized as net periodic benefit cost						5.0				5.0
Amount recognized as cumulative foreign currency translation during the period							(2.9)			(2.9)
Dividends to shareholders								(48.9)
Total accumulated other comprehensive income										36.5
Net shares reissued at cost under employee stock-based compensation plans									23.9
Stock-based compensation								(3.1)
Balance at end of period at Sep. 30, 2014		$ 0.6	1,827.4	$ 214.1	293.7	(71.1)	(8.5)	1,489.2	(759.4)	2,771.9
Balance at beginning of period at Dec. 31, 2014			1,830.7		300.9	(84.3)	(10.2)	1,558.7	(752.4)	2,844.0
Net income								253.9		253.9
Shares purchased at cost									(85.3)
Employee and director stock-based awards and other			2.8
Net (depreciation) appreciation on available-for-sale securities and derivative instruments					(103.0)					(103.0)
Net amount arising in the period						(1.4)				(1.4)
Net amount recognized as net periodic benefit cost						6.8				5.4
Amount recognized as cumulative foreign currency translation during the period							(4.8)			(4.8)
Dividends to shareholders								(54.3)
Total accumulated other comprehensive income										(102.4)
Net shares reissued at cost under employee stock-based compensation plans									27.3
Stock-based compensation								(8.5)
Balance at end of period at Sep. 30, 2015		$ 0.6	$ 1,833.5	$ 104.0	$ 197.9	$ (78.9)	$ (15.0)	$ 1,749.8	$ (810.4)	$ 2,877.5