Investments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fixed maturities
Available-for-sale securities disclosures
Amortized cost	$ 47,906.6	$ 47,715.3
Gross unrealized gains	3,937.1	4,095.1
Gross unrealized losses	759.6	871.1
Fair value	51,084.1	50,939.3
Other-than-temporary impairments in AOCI	322.2	302.0
Net unrealized gains (losses) on impaired fixed maturities, available-for-sale related to changes in fair value subsequent to the impairment date	127.9	95.0
U.S. government and agencies
Available-for-sale securities disclosures
Amortized cost	930.4	911.4
Gross unrealized gains	29.0	33.2
Gross unrealized losses	6.1	0.3
Fair value	953.3	944.3
Non-U.S. governments
Available-for-sale securities disclosures
Amortized cost	903.9	944.9
Gross unrealized gains	278.9	264.3
Gross unrealized losses	3.1	0.9
Fair value	1,179.7	1,208.3
States and political subdivisions
Available-for-sale securities disclosures
Amortized cost	3,260.8	2,940.4
Gross unrealized gains	234.8	241.1
Gross unrealized losses	3.8	2.7
Fair value	3,491.8	3,178.8
Corporate debt securities
Available-for-sale securities disclosures
Amortized cost	31,510.2	31,615.4
Gross unrealized gains	2,881.9	3,029.9
Gross unrealized losses	289.3	319.9
Fair value	34,102.8	34,325.4
Other-than-temporary impairments in AOCI	20.4	19.5
Residential mortgage-backed pass-through securities
Available-for-sale securities disclosures
Amortized cost	2,950.3	3,022.7
Gross unrealized gains	176.8	204.4
Gross unrealized losses	3.9	0.4
Fair value	3,123.2	3,226.7
Commercial mortgage-backed securities
Available-for-sale securities disclosures
Amortized cost	4,110.9	4,094.8
Gross unrealized gains	251.7	241.7
Gross unrealized losses	371.9	439.1
Fair value	3,990.7	3,897.4
Other-than-temporary impairments in AOCI	217.8	195.4
Collateralized debt obligations
Available-for-sale securities disclosures
Amortized cost	431.7	428.8
Gross unrealized gains	8.6	7.0
Gross unrealized losses	46.3	56.6
Fair value	394.0	379.2
Other-than-temporary impairments in AOCI	3.1	4.3
Other debt obligations
Available-for-sale securities disclosures
Amortized cost	3,808.4	3,756.9
Gross unrealized gains	75.4	73.5
Gross unrealized losses	35.2	51.2
Fair value	3,848.6	3,779.2
Other-than-temporary impairments in AOCI	80.9	82.8
Equity securities
Available-for-sale securities disclosures
Amortized cost	138.9	132.4
Gross unrealized gains	14.1	12.6
Gross unrealized losses	6.0	8.5
Fair value	$ 147.0	$ 136.5

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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