Schedule of New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Consolidated Statements of Income
Net Investment Income	$ 612.1	$ 611.3	$ 1,844.8	$ 1,859.5
Income Tax - Current	95.1	66.8	241.6	191.2
Income Tax - Deferred	0.0	31.1	39.1	121.1
Net Income	$ 203.8	$ 219.1	$ 641.0	$ 684.3
Basic Earnings Per Share	$ 0.83	$ 0.86	$ 2.58	$ 2.67
Diluted Earnings Per Share	$ 0.83	$ 0.86	$ 2.57	$ 2.66
Consolidated Statements of Cash Flows
Change in Income Taxes			$ 164.0	$ 208.4
Non-cash Components of Net Investment Income			(129.9)	(140.9)
Consolidated Balance Sheets
Other Long-term Investments	$ 590.2	$ 516.7	590.2	516.7	$ 545.0
Deferred Income Tax	73.4	418.0	73.4	418.0	62.0
Retained Earnings	$ 7,814.1	8,630.6	$ 7,814.1	8,630.6	7,302.3
Previously Reported Amount
Consolidated Statements of Income
Net Investment Income		606.4		1,848.0
Income Tax - Current		58.8		167.1
Income Tax - Deferred		32.2		125.5
Net Income		$ 221.1		$ 692.5
Basic Earnings Per Share		$ 0.87		$ 2.70
Diluted Earnings Per Share		$ 0.87		$ 2.69
Consolidated Statements of Cash Flows
Change in Income Taxes				$ 188.7
Non-cash Components of Net Investment Income				(129.4)
Consolidated Balance Sheets
Other Long-term Investments		$ 558.8		558.8	591.9
Deferred Income Tax		432.7		432.7	78.4
Retained Earnings		8,658.0		8,658.0	7,332.8
Accounting Standards Update 2014-01 | Restatement Adjustment
Consolidated Statements of Income
Net Investment Income		4.9		11.5
Income Tax - Current		8.0		24.1
Income Tax - Deferred		(1.1)		(4.4)
Net Income		$ (2.0)		$ (8.2)
Basic Earnings Per Share		$ (0.01)		$ (0.03)
Diluted Earnings Per Share		$ (0.01)		$ (0.03)
Consolidated Statements of Cash Flows
Change in Income Taxes				$ 19.7
Non-cash Components of Net Investment Income				(11.5)
Consolidated Balance Sheets
Other Long-term Investments		$ (42.1)		(42.1)	(46.9)
Deferred Income Tax		(14.7)		(14.7)	(16.4)
Retained Earnings		$ (27.4)		$ (27.4)	$ (30.5)	$ 19.2