Fair Value Measurements - Other Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 20,439.7		$ 21,378.3
Derivative assets	430.5		512.7
Derivatives liabilities	306.3		346.8
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	181.3		179.6
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	757.7		1,229.3
Derivative assets	430.5		512.7
Notes receivable from affiliates	175.0		175.0
Assets held in separate accounts	58,022.3		53,655.3
Derivatives liabilities	306.3		346.8
Long-term debt, fair value	4.9		4.9
Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	181.3		179.6
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	757.7		1,229.3
Derivative assets	430.5		512.7
Notes receivable from affiliates	185.0		194.3
Assets held in separate accounts	58,022.3		53,655.3
Derivatives liabilities	306.3		346.8
Long-term debt, fair value	4.9		4.9
Investment Contract, without a fixed maturity | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	20,606.2	[1]	20,263.4	[1]
Investment Contract, without a fixed maturity | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	24,052.7	[1]	25,156.5	[1]
Within fixed maturity investments | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	653.7		680.0
Within fixed maturity investments | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	755.1		837.3
FIA | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	21.1		20.4
FIA | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	21.1		20.4
Stabilizer and MCGs | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	28.0		102.0
Stabilizer and MCGs | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	28.0		102.0
Mortgage loans on real estate | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,187.9		2,872.7
Mortgage loans on real estate | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	3,210.1		2,946.9
Policy loans | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	239.8		240.9
Policy loans | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	239.8		240.9
Fixed maturities, available-for-sale, including securities pledged | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	20,530.4		21,455.2
Fixed maturities, available-for-sale, including securities pledged | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	20,530.4		21,455.2
Equity securities, available-for-sale | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	126.7		142.8
Equity securities, available-for-sale | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	126.7		142.8
Reinsurance Agreements | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	(45.8)		0
Reinsurance Agreements | Total
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives liabilities	$ (45.8)		$ 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.