Fair Value Measurements - Other Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	$ 73,984.0		$ 71,567.5
Loans	6,262.9		4,965.3
Derivatives	1,136.8		1,149.3
Derivatives	656.8		1,351.8
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	348.2		236.4
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,675.4		4,441.8
Derivatives	1,136.8		1,149.3
Other investments	106.8		124.6
Assets held in separate accounts	107,059.3		106,827.1
Other derivatives	656.8		1,351.8
Long-term debt	3,515.5		3,514.7
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Limited partnerships/corporations	348.2		236.4
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,675.4		4,441.8
Derivatives	1,136.8		1,149.3
Other investments	106.8		131.1
Assets held in separate accounts	107,059.3		106,827.1
Other derivatives	656.8		1,351.8
Long-term debt	3,851.3		3,717.8
Stabilizer (Investment Only) and MCG Contracts [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of Plans	88.00%		88.00%
Funding agreements without fixed maturities and deferred annuities(1) | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	49,440.6		49,418.4
Funding agreements without fixed maturities and deferred annuities(1) | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	53,877.9		53,713.8
Funding agreements with fixed maturities and guaranteed investment contracts | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,749.3		2,692.3
Funding agreements with fixed maturities and guaranteed investment contracts | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,733.1		2,663.9
Supplementary contracts, immediate annuities and other | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	3,778.6		3,383.6
Supplementary contracts, immediate annuities and other | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	4,054.8		3,567.3
FIA | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,894.2		1,736.7
FIA | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,894.2		1,736.7
GMAB/GMWB/GMWBL | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,157.2		908.9
GMAB/GMWB/GMWBL | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,157.2		908.9
Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of Plans	100.00%		100.00%
Stabilizer and MCGs | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	45.0		0
Stabilizer and MCGs | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	45.0		0
Stabilizer with Recordkeeping Agreements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Percentage of Plans	12.00%		12.00%
Mortgage loans on real estate | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	9,949.7		9,312.2
Mortgage loans on real estate | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	10,268.5		9,404.7
Policy loans | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	2,104.2		2,147.0
Policy loans | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans	2,104.2		2,147.0
Fixed maturities, available-for-sale, including securities pledged | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	74,735.1		72,718.8
Fixed maturities, available-for-sale, including securities pledged | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	74,735.1		72,718.8
Equity securities, available-for-sale | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	270.5		314.4
Equity securities, available-for-sale | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments	270.5		314.4
Reinsurance agreements | Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	123.5		79.0
Reinsurance agreements | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivatives	123.5		79.0
Level 1 | Measured at fair value on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,555.5		4,396.9
Assets held in separate accounts	102,248.3		101,437.5
Level 1 | Measured at fair value on a recurring basis | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0		0
Level 1 | Measured at fair value on a recurring basis | GMAB/GMWB/GMWBL
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0	[1]	0
Level 1 | Measured at fair value on a recurring basis | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0		0
Level 2 | Measured at fair value on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	119.9		44.9
Assets held in separate accounts	4,797.3		5,376.5
Level 2 | Measured at fair value on a recurring basis | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0		0
Level 2 | Measured at fair value on a recurring basis | GMAB/GMWB/GMWBL
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0	[1]	0
Level 2 | Measured at fair value on a recurring basis | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	0		0
Level 3 | Measured at fair value on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	0		0
Assets held in separate accounts	13.7		13.1
Level 3 | Measured at fair value on a recurring basis | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,894.2		1,736.7
Level 3 | Measured at fair value on a recurring basis | GMAB/GMWB/GMWBL
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,157.2	[1]	908.9
Level 3 | Measured at fair value on a recurring basis | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	45.0		0
Fair Value | Measured at fair value on a recurring basis
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securities loan agreement	3,675.4		4,441.8
Assets held in separate accounts	107,059.3		106,827.1
Fair Value | Measured at fair value on a recurring basis | FIA
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,894.2		1,736.7
Fair Value | Measured at fair value on a recurring basis | GMAB/GMWB/GMWBL
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	1,157.2		908.9
Fair Value | Measured at fair value on a recurring basis | Stabilizer and MCGs
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liabilities	$ 45.0		$ 0

[1] Guaranteed minimum accumulation benefits ("GMAB"), Guaranteed minimum withdrawal benefits ("GMWB") and Guaranteed minimum withdrawal benefits with life payouts ("GMWBL").