Investments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fixed maturities
Available-for-sale securities disclosures
Amortized cost	$ 47,370.7	$ 47,700.2
Gross unrealized gains	3,159.9	2,424.9
Gross unrealized losses	1,239.3	1,154.3
Other-than-temporary impairments in AOCI	284.6	334.5
Fair value	49,006.7	48,636.3
Net unrealized gains (losses) on impaired fixed maturities, available-for-sale related to changes in fair value subsequent to the impairment date	28.9	58.6
U.S. government and agencies
Available-for-sale securities disclosures
Amortized cost	772.3	748.5
Gross unrealized gains	32.8	21.0
Gross unrealized losses		0.2
Fair value	805.1	769.3
Non-U.S. governments
Available-for-sale securities disclosures
Amortized cost	917.6	744.7
Gross unrealized gains	180.5	127.9
Gross unrealized losses	1.4
Fair value	1,096.7	872.6
States and political subdivisions
Available-for-sale securities disclosures
Amortized cost	2,670.0	2,615.0
Gross unrealized gains	218.2	64.7
Gross unrealized losses	5.5	23.3
Fair value	2,882.7	2,656.4
Corporate debt securities
Available-for-sale securities disclosures
Amortized cost	31,954.1	32,523.8
Gross unrealized gains	2,321.3	1,913.7
Gross unrealized losses	699.5	527.0
Other-than-temporary impairments in AOCI	19.4	18.0
Fair value	33,556.5	33,892.5
Residential mortgage-backed pass-through securities
Available-for-sale securities disclosures
Amortized cost	3,155.8	3,077.9
Gross unrealized gains	187.9	124.2
Gross unrealized losses	0.7	5.9
Fair value	3,343.0	3,196.2
Commercial mortgage-backed securities
Available-for-sale securities disclosures
Amortized cost	3,894.3	4,424.9
Gross unrealized gains	117.0	118.0
Gross unrealized losses	429.4	506.1
Other-than-temporary impairments in AOCI	168.2	194.6
Fair value	3,413.7	3,842.2
Collateralized debt obligations
Available-for-sale securities disclosures
Amortized cost	399.7	380.5
Gross unrealized gains	1.9	1.7
Gross unrealized losses	55.8	51.8
Other-than-temporary impairments in AOCI	7.0	37.4
Fair value	338.8	293.0
Other debt obligations
Available-for-sale securities disclosures
Amortized cost	3,606.9	3,184.9
Gross unrealized gains	100.3	53.7
Gross unrealized losses	47.0	40.0
Other-than-temporary impairments in AOCI	90.0	84.5
Fair value	3,570.2	3,114.1
Equity securities
Available-for-sale securities disclosures
Amortized cost	74.9	180.0
Gross unrealized gains	8.7	8.1
Gross unrealized losses	6.5	18.2
Fair value	$ 77.1	$ 169.9

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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