Significant Accounting Policies Schedule of New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Consolidated Statements of Income
Net Investment Income	$ 636.4	$ 612.1	$ 630.7	$ 602.0	$ 632.7	$ 611.3	$ 632.4	$ 615.8	$ 2,481.2	$ 2,492.2	$ 2,506.9
Income Tax - Current									342.1	135.5	328.5
Income Tax - Deferred									29.1	4.4	44.5
Net Income	$ 226.1	$ 203.8	$ 224.3	$ 212.9	$ (282.2)	$ 219.1	$ 239.4	$ 225.8	$ 867.1	$ 402.1	$ 847.0
Basic Earnings Per Share	$ 0.93	$ 0.83	$ 0.90	$ 0.85	$ (1.12)	$ 0.86	$ 0.93	$ 0.87	$ 3.51	$ 1.57	$ 3.20
Diluted Earnings Per Share	$ 0.93	$ 0.83	$ 0.90	$ 0.84	$ (1.12)	$ 0.86	$ 0.93	$ 0.86	$ 3.50	$ 1.57	$ 3.19
Consolidated Statements of Stockholders' Equity
Retained Earnings	$ 7,995.2				$ 7,302.3				$ 7,995.2	$ 7,302.3	$ 8,064.0	$ 7,363.5
Consolidated Statements of Cash Flows
Change in Income Taxes									180.5	(18.6)	(23.5)
Non-cash Components of Net Investment Income									(194.1)	(195.7)	(226.3)
Consolidated Balance Sheets
Other Long-term Investments	583.0				545.0				583.0	545.0
Deferred Income Tax	$ 91.8				62.0				$ 91.8	62.0
Previously Reported
Consolidated Statements of Income
Net Investment Income										2,477.4	2,492.1
Income Tax - Current										103.3	296.6
Income Tax - Deferred										10.5	50.5
Net Income										$ 413.4	$ 858.1
Basic Earnings Per Share										$ 1.62	$ 3.24
Diluted Earnings Per Share										$ 1.61	$ 3.23
Consolidated Statements of Stockholders' Equity
Retained Earnings					7,332.8					$ 7,332.8	$ 8,083.2	7,371.6
Consolidated Statements of Cash Flows
Change in Income Taxes										(44.7)	(49.4)
Non-cash Components of Net Investment Income										(180.9)	(211.5)
Consolidated Balance Sheets
Other Long-term Investments					591.9					591.9
Deferred Income Tax					78.4					78.4
Accounting Standards Update 2014-01 | Restatement Adjustment
Consolidated Statements of Income
Net Investment Income										14.8	14.8
Income Tax - Current										32.2	31.9
Income Tax - Deferred										(6.1)	(6.0)
Net Income										$ (11.3)	$ (11.1)
Basic Earnings Per Share										$ (0.05)	$ (0.04)
Diluted Earnings Per Share										$ (0.04)	$ (0.04)
Consolidated Statements of Stockholders' Equity
Retained Earnings					(30.5)					$ (30.5)	$ (19.2)	$ (8.1)
Consolidated Statements of Cash Flows
Change in Income Taxes										26.1	25.9
Non-cash Components of Net Investment Income										(14.8)	$ (14.8)
Consolidated Balance Sheets
Other Long-term Investments					(46.9)					(46.9)
Deferred Income Tax					$ (16.4)					$ (16.4)