Fair Value Measurements - Other Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities, available-for-sale	$ 82.3	$ 131.3
Investments	24,209.4	22,140.9
Derivative assets	470.8	450.3
Assets held in separate accounts	61,980.7	58,910.6
Derivatives liabilities	172.9	115.1
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities including securities pledged, Fair Value	24,895.0	22,258.8
Equity securities, available-for-sale	82.3	131.3
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	972.7	902.6
Derivative assets	470.8	450.3
Notes receivable from affiliates	175.0	175.0
Assets held in separate accounts	61,980.7	58,910.6
Deposit liabilities	172.0	194.8
Derivatives liabilities	172.9	115.1
Long-term debt, fair value	4.9	4.9
Carrying Value | Embedded derivative on reinsurance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Embedded derivative on reinsurance	(43.5)	(71.6)
Carrying Value | Funding agreements without fixed maturities and deferred annuities(1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	24,942.7	22,979.4
Carrying Value | Supplementary contracts, immediate annuities and other
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	386.1	411.8
Carrying Value | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	23.3	23.1
Carrying Value | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	150.5	161.3
Carrying Value | Mortgage loans on real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	4,254.5	3,729.1
Carrying Value | Policy loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	218.9	229.8
Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities including securities pledged, Fair Value	24,895.0	22,258.8
Equity securities, available-for-sale	82.3	131.3
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	972.7	902.5
Derivative assets	470.8	450.3
Notes receivable from affiliates	216.2	208.4
Assets held in separate accounts	61,980.7	58,910.6
Deposit liabilities	245.8	194.8
Derivatives liabilities	172.9	115.1
Long-term debt, fair value	4.9	4.9
Total | Embedded derivative on reinsurance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Embedded derivative on reinsurance	(43.5)	(71.6)
Total | Funding agreements without fixed maturities and deferred annuities(1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	28,876.5	27,612.3
Total | Supplementary contracts, immediate annuities and other
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	437.5	479.2
Total | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	23.3	23.1
Total | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	150.5	161.3
Total | Mortgage loans on real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	4,339.6	3,881.1
Total | Policy loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 218.9	$ 229.8