Fair Value Measurements - Fair Vaue Measurement (Details) (USD $)	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	$ 21,500,000,000		$ 19,200,000,000
Equity securities, available-for-sale	142,800,000		144,900,000
Derivatives	512,700,000		446,600,000
Fixed maturities valued using unadjusted broker quotes	175,500,000		194,900,000
Fixed maturities valued using unadjusted prices	16,700,000,000		14,800,000,000
Transfers of securities between Level 1 and Level 2	0		0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	1,093,400,000		1,180,300,000
Equity securities, available-for-sale	125,800,000
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	1,229,300,000		953,900,000
Assets held in separate accounts	47,916,500,000		40,556,800,000
Total assets, fair value	50,365,400,000		42,822,600,000
Total liabilities, fair value	700,000		0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	20,140,300,000		17,811,000,000
Equity securities, available-for-sale	0
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	0		4,800,000
Assets held in separate accounts	5,722,500,000		4,722,300,000
Total assets, fair value	26,375,100,000		22,979,000,000
Total liabilities, fair value	346,100,000		347,400,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	221,500,000		248,900,000
Equity securities, available-for-sale	17,000,000
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	0		0
Assets held in separate accounts	16,300,000		16,100,000
Total assets, fair value	254,800,000		284,000,000
Total liabilities, fair value	122,400,000		249,900,000
Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	21,455,200,000		19,240,200,000
Equity securities, available-for-sale	142,800,000
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	1,229,300,000		958,700,000
Assets held in separate accounts	53,655,300,000		45,295,200,000
Total assets, fair value	76,995,300,000		66,085,600,000
Total liabilities, fair value	469,200,000		597,300,000
Interest rate contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		5,700,000
Derivatives, fair value	700,000		0
Interest rate contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	508,300,000		437,600,000
Derivatives, fair value	327,800,000		306,400,000
Interest rate contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives, fair value	0		0
Interest rate contracts | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	508,300,000		443,300,000
Derivatives, fair value	328,500,000		306,400,000
Foreign exchange contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives, fair value	0		0
Foreign exchange contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	400,000		700,000
Derivatives, fair value	18,300,000		32,400,000
Foreign exchange contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives, fair value	0		0
Foreign exchange contracts | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	400,000		700,000
Derivatives, fair value	18,300,000		32,400,000
Equity contract | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	400,000
Equity contract | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0
Equity contract | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0
Equity contract | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	400,000
Credit contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives, fair value	0		0
Credit contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	3,600,000		2,600,000
Derivatives, fair value	0		8,600,000
Credit contracts | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0		0
Derivatives, fair value	0		12,600,000
Credit contracts | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	3,600,000		2,600,000
Derivatives, fair value	0		21,200,000
Stabilizer and MCGs | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	102,000,000
Stabilizer and MCGs | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
Stabilizer and MCGs | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
Stabilizer and MCGs | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	102,000,000		221,000,000
Stabilizer and MCGs | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			221,000,000
FIA | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	20,400,000
FIA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
FIA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	0		0
FIA | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities	20,400,000		16,300,000
FIA | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities			16,300,000
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	21,455,200,000		19,240,200,000
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	1,146,600,000		1,231,600,000
U.S. Treasuries | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	1,093,400,000		1,180,300,000
U.S. Treasuries | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	53,200,000		51,300,000
U.S. Treasuries | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
U.S. Treasuries | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	1,146,600,000		1,231,600,000
U.S. government agencies and authorities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	397,000,000		410,700,000
U.S. government agencies and authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
U.S. government agencies and authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	397,000,000		410,700,000
U.S. government agencies and authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
U.S. government agencies and authorities | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	397,000,000		410,700,000
U.S. corporate, state and municipalities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
U.S. corporate, state and municipalities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	10,512,800,000		8,883,500,000
U.S. corporate, state and municipalities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	154,600,000		129,100,000
U.S. corporate, state and municipalities | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	10,667,400,000		9,012,600,000
Foreign securities government
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	496,000,000	[1]	345,200,000	[1]
Foreign securities government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0	[1]	0	[1]
Foreign securities government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	5,527,400,000	[1]	4,937,000,000	[1]
Foreign securities government | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	24,600,000	[1]	51,100,000	[1]
Foreign securities government | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	5,552,000,000	[1]	4,988,100,000	[1]
Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	2,357,500,000		2,247,100,000
Residential mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
Residential mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	2,348,400,000		2,206,100,000
Residential mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	9,100,000		41,000,000
Residential mortgage-backed | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	2,357,500,000		2,247,100,000
Commercial mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	839,100,000		911,300,000
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	839,100,000		911,300,000
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
Commercial mortgage-backed | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	839,100,000		911,300,000
Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	0		0
Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	462,400,000		411,100,000
Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	33,200,000		27,700,000
Asset-backed Securities | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	495,600,000		438,800,000
Equity securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale			125,900,000
Equity securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale			0
Equity securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale			19,000,000
Equity securities | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, available-for-sale			$ 144,900,000

[1] Primarily U.S. dollar denominated.