Fair Value Measurements - Other Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 21,125.1		$ 20,700.6
Derivative assets	412.9		464.4
Derivatives liabilities	179.0		216.6
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	172.4		180.9
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	762.0		529.7
Derivative assets	412.9		464.4
Notes receivable from affiliates	175.0		175.0
Assets held in separate accounts	61,857.7		60,104.9
Derivatives liabilities	179.0		216.6
Long-term debt, fair value	4.9		4.9
Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	172.4		180.9
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	762.0		529.7
Derivative assets	412.9		464.4
Notes receivable from affiliates	199.5		186.4
Assets held in separate accounts	61,857.7		60,104.9
Derivatives liabilities	179.0		216.6
Long-term debt, fair value	4.9		4.9
Funding agreements without fixed maturities and deferred annuities(1) | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	21,024.3	[1]	21,010.8	[1]
Funding agreements without fixed maturities and deferred annuities(1) | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	24,833.1	[1]	24,379.6	[1]
Supplementary contracts, immediate annuities and other | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	614.1		624.3
Supplementary contracts, immediate annuities and other | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	716.0		727.1
FIA | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	24.3		23.1
FIA | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	24.3		23.1
Stabilizer and MCGs | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	18.0		0
Stabilizer and MCGs | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	18.0		0
Mortgage loans on real estate | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,377.0		3,396.1
Mortgage loans on real estate | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,383.2		3,403.9
Policy loans | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	240.7		242.0
Policy loans | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	240.7		242.0
Fixed maturities, available-for-sale, including securities pledged | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	21,209.9		20,705.8
Fixed maturities, available-for-sale, including securities pledged | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	21,209.9		20,705.8
Equity securities, available-for-sale | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	127.0		134.9
Equity securities, available-for-sale | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	127.0		134.9
Reinsurance Agreements | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	(30.7)		(54.0)
Reinsurance Agreements | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	$ (30.7)		$ (54.0)

[1] Certain amounts included in Funding agreements without fixed maturities and deferred annuities are also reflected within the Annuity product guarantees section of the table above.