Investments - Net Investment Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Major categories of net investment income
Gross investment income (loss)	$ 3,548.6	$ 3,380.9	$ 3,129.1
Investment expenses	(89.3)	(84.4)	(77.0)
Net investment income (loss)	3,459.3	3,296.5	3,052.1
Mortgage loans
Major categories of net investment income
Gross investment income (loss)	605.9	573.9	575.1
Real estate
Major categories of net investment income
Gross investment income (loss)	129.4	127.9	97.1
Policy loans
Major categories of net investment income
Gross investment income (loss)	45.6	46.3	46.3
Cash and cash equivalents
Major categories of net investment income
Gross investment income (loss)	28.6	14.2	8.5
Derivatives
Major categories of net investment income
Gross investment income (loss)	(3.2)	(36.1)	(66.6)
Other investment types
Major categories of net investment income
Gross investment income (loss)	319.6	329.2	265.3
Available-for-sale \| Fixed maturities
Major categories of net investment income
Gross investment income (loss)	2,342.3	2,253.8	2,131.8
Available-for-sale \| Equity portfolios
Major categories of net investment income
Gross investment income (loss)	21.7	20.1	15.0
Trading \| Fixed maturities
Major categories of net investment income
Gross investment income (loss)	12.6	20.2	21.4
Trading \| Equity portfolios
Major categories of net investment income
Gross investment income (loss)	$ 46.1	$ 31.4	$ 35.2

Income derived from investments and on cash and cash equivalents, net of amortization of purchase discounts and premiums related to investments in debt securities. Interest income represents earnings which reflect the time value of money or transactions in which the payments are for the use or forbearance of money. Dividend income represents a distribution of earnings to shareholders by investee companies. The purchase discount or premium is amortized to income or expense, respectively, over the life (holding period) of the securities to arrive at an amount of periodic interest income or expense which results in a constant effective yield on the investments.

No definition available.

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16(Column G))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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