Fair Value Measurements - Other Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities, available-for-sale	$ 85.2	$ 82.3
Derivative assets	168.2	470.8
Assets held in separate accounts	68,373.3	61,980.7
Derivatives liabilities	51.9	172.9
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, including securities pledged	24,798.9	24,895.0
Equity securities, available-for-sale	85.2	82.3
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	725.9	972.7
Derivative assets	168.2	470.8
Notes receivable from affiliates	175.0	175.0
Assets held in separate accounts	68,373.3	61,980.7
Deposit liabilities	158.4	172.0
Derivatives liabilities	51.9	172.9
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	(27.6)	(43.5)
Carrying Value | Funding agreements without fixed maturities and deferred annuities(1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	25,255.6	24,942.7
Carrying Value | Supplementary contracts, immediate annuities and other
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	371.9	386.1
Carrying Value | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	23.0	23.3
Carrying Value | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	141.2	150.5
Carrying Value | Mortgage loans on real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	4,848.2	4,254.5
Carrying Value | Policy loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	213.6	218.9
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, including securities pledged	24,798.9	24,895.0
Equity securities, available-for-sale	85.2	82.3
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	725.9	972.7
Derivative assets	168.2	470.8
Notes receivable from affiliates	225.1	216.2
Assets held in separate accounts	68,373.3	61,980.7
Deposit liabilities	214.8	245.8
Derivatives liabilities	51.9	172.9
Long-term debt, fair value	4.9	4.9
Fair Value | Embedded derivative on reinsurance
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Embedded derivative on reinsurance	(27.6)	(43.5)
Fair Value | Funding agreements without fixed maturities and deferred annuities(1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	29,727.2	28,876.5
Fair Value | Supplementary contracts, immediate annuities and other
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	436.3	437.5
Fair Value | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	23.0	23.3
Fair Value | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	141.2	150.5
Fair Value | Mortgage loans on real estate
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	4,913.7	4,339.6
Fair Value | Policy loans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	$ 213.6	$ 218.9