Financial Instruments Fair Vaue Measurement (Details) (USD $)	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	$ 19,900,000,000
Equity securities, available-for-sale	144,700,000		144,900,000
Derivatives	532,300,000		446,600,000
Fixed Maturities Valued Using Unadjusted Broker Quotes	125,800,000
Fixed Maturities Valued Using Unadjusted Prices	15,300,000,000
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	1,192,000,000		953,900,000
Assets held in separate accounts	44,318,200,000		40,556,800,000
Assets, Fair Value Disclosure, Recurring	46,640,100,000		42,822,600,000
Liabilities, Fair Value Disclosure, Recurring	0		0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	200,000		4,800,000
Assets held in separate accounts	4,978,800,000		4,722,300,000
Assets, Fair Value Disclosure, Recurring	24,172,200,000		22,979,000,000
Liabilities, Fair Value Disclosure, Recurring	363,000,000		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]
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	0		0
Assets held in separate accounts	8,700,000		16,100,000
Assets, Fair Value Disclosure, Recurring	234,300,000		284,000,000
Liabilities, Fair Value Disclosure, Recurring	150,600,000		250,000,000
Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, short-term investments and short-term investments under securirites loan agreement	1,192,200,000		958,700,000
Assets held in separate accounts	49,305,700,000		45,295,200,000
Assets, Fair Value Disclosure, Recurring	71,046,600,000		66,085,600,000
Liabilities, Fair Value Disclosure, Recurring	513,600,000		597,400,000
Product guarantees | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	0		0
Product guarantees | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	0		0
Product guarantees | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	133,000,000		221,000,000		2,500,000	(2,000,000)	3,000,000
Product guarantees | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	133,000,000		221,000,000
Fixed indexed annuities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	0		0
Fixed indexed annuities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	0		0
Fixed indexed annuities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	17,600,000		16,300,000		11,800,000	8,400,000	5,600,000
Fixed indexed annuities | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives, Fair Value Disclosure	17,600,000		16,300,000
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	3,100,000		5,700,000
Derivatives, Fair Value Disclosure	0		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	525,000,000		437,600,000
Derivatives, Fair Value Disclosure	337,700,000		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 Disclosure	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	528,100,000		443,300,000
Derivatives, Fair Value Disclosure	337,700,000		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 Disclosure	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	2,700,000		700,000
Derivatives, Fair Value Disclosure	23,600,000		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 Disclosure	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	2,700,000		700,000
Derivatives, Fair Value Disclosure	23,600,000		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
Derivatives, Fair Value Disclosure	0
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
Derivatives, Fair Value Disclosure	1,700,000
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
Derivatives, Fair Value Disclosure	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
Derivatives, Fair Value Disclosure	1,700,000
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 Disclosure	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	1,100,000		2,600,000
Derivatives, Fair Value Disclosure	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 Disclosure	0		12,700,000
Credit contracts | Fair Value, Measurements, Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	1,100,000		2,600,000
Derivatives, Fair Value Disclosure	0		21,300,000
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	19,871,700,000		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	1,053,500,000		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	1,000,200,000		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	53,300,000		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	1,053,500,000		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	403,400,000		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	403,400,000		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	403,400,000		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	9,529,200,000		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	151,600,000		129,100,000		133,500,000	13,900,000	11,200,000
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	9,680,800,000		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	453,400,000	[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		0
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	5,109,100,000		4,937,000,000
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	19,400,000		51,100,000		24,900,000	7,600,000	11,400,000
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	5,128,500,000		4,988,100,000
Residential mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, including securities pledged, Fair Value	2,257,200,000		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	2,251,200,000		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	6,000,000		41,000,000		12,300,000	119,900,000	254,700,000
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	2,257,200,000		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	891,900,000		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	891,900,000		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	891,900,000		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	426,300,000		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	30,100,000		27,700,000		49,100,000	52,000,000	247,700,000
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	456,400,000		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	126,200,000		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		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	18,500,000		19,000,000		21,700,000	29,800,000	27,700,000
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	$ 144,700,000		$ 144,900,000

[1] Primarily U.S. dollar denominated.