Fair Value Measurements - Other Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 21,414.2		$ 20,700.6
Derivative assets	445.8		464.4
Derivatives liabilities	193.9		216.6
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	228.9		180.9
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	670.0		529.7
Derivative assets	445.8		464.4
Notes receivable from affiliates	175.0		175.0
Assets held in separate accounts	64,362.4		60,104.9
Derivatives liabilities	193.9		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	228.9		180.9
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	670.0		529.7
Derivative assets	445.8		464.4
Notes receivable from affiliates	207.4		186.4
Assets held in separate accounts	64,362.4		60,104.9
Derivatives liabilities	193.9		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	21,128.2	[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	25,610.2	[1]	24,379.6	[1]
Supplementary contracts, immediate annuities and other | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	599.9		624.3
Supplementary contracts, immediate annuities and other | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	711.1		727.1
FIA | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	25.7		23.1
FIA | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	25.7		23.1
Stabilizer and MCGs | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	32.0		0
Stabilizer and MCGs | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	32.0		0
Mortgage loans on real estate | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,434.4		3,396.1
Mortgage loans on real estate | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,476.7		3,403.9
Policy loans | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	239.6		242.0
Policy loans | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	239.6		242.0
Fixed maturities, available-for-sale, including securities pledged | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	21,527.5		20,705.8
Fixed maturities, available-for-sale, including securities pledged | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	21,527.5		20,705.8
Equity securities, available-for-sale | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	123.1		134.9
Equity securities, available-for-sale | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	123.1		134.9
Reinsurance Agreements | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	(12.6)		(54.0)
Reinsurance Agreements | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	$ (12.6)		$ (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.