Derivative Financial Instruments - Net Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (70.0)	$ 51.0
Net Investment Income	394.0	382.0
Other net realized capital gains (losses)	(4.0)	(78.0)
Other Net Realized Capital Gains (Losses) [Member] \| Fixed maturities
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0	(2.0)
Other Net Realized Capital Gains (Losses) [Member] \| Product
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	20.0
Interest Credited and Other Benefits to Contract Owners \| Reinsurance agreements
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(38.0)	29.0
Designated as Hedging Instrument \| Cash flow hedges \| Other Net Realized Capital Gains (Losses) [Member] \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.0
Designated as Hedging Instrument \| Cash flow hedges \| Other Net Realized Capital Gains (Losses) [Member] \| Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.0
Designated as Hedging Instrument \| Cash flow hedges \| Other Comprehensive Income (Loss) [Member] \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0
Designated as Hedging Instrument \| Cash flow hedges \| Other Comprehensive Income (Loss) [Member] \| Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(8.0)
Designated as Hedging Instrument \| Cash flow hedges \| Net Investment Income [Member] \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.0
Designated as Hedging Instrument \| Cash flow hedges \| Net Investment Income [Member] \| Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	2.0
Designated as Hedging Instrument \| Cash flow hedges \| Investment Income [Member] \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	0.0
Designated as Hedging Instrument \| Cash flow hedges \| Investment Income [Member] \| Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Net realized gains (losses) on derivatives	2.0
Not Designated as Hedging Instrument \| Other Net Realized Capital Gains (Losses) [Member] \| Credit contracts
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0	0.0
Not Designated as Hedging Instrument \| Other Net Realized Capital Gains (Losses) [Member] \| Interest rate contracts
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(36.0)	5.0
Not Designated as Hedging Instrument \| Other Net Realized Capital Gains (Losses) [Member] \| Foreign exchange contracts
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1.0	(1.0)
Not Designated as Hedging Instrument \| Other Net Realized Capital Gains (Losses) [Member] \| Equity contract
Derivatives, Fair Value [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ 1.0	$ 0.0

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624177-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount of gain (loss) included in earnings for investments classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

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- Definition

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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=114873790&loc=SL114874131-224263

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