Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 38,187.2	$ 37,751.5
Gross Unrealized Gain	4,698.0	7,277.5
Gross Unrealized Loss	287.6	56.0
Fixed Maturity Securities	42,597.6	44,973.0
United States Government and Government Agencies and Authorities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,020.8	1,020.9
Gross Unrealized Gain	206.7	329.0
Gross Unrealized Loss	4.3	1.1
Fixed Maturity Securities	1,223.2	1,348.8
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,675.5	1,498.4
Gross Unrealized Gain	131.2	316.2
Gross Unrealized Loss	36.9	7.8
Fixed Maturity Securities	1,769.8	1,806.8
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,203.1	1,280.4
Gross Unrealized Gain	174.2	226.6
Gross Unrealized Loss	2.0	0
Fixed Maturity Securities	1,375.3	1,507.0
Public Utilities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,372.1	9,294.3
Gross Unrealized Gain	1,233.7	1,865.0
Gross Unrealized Loss	49.1	15.1
Fixed Maturity Securities	10,556.7	11,144.2
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,947.0	1,927.9
Gross Unrealized Gain	216.0	289.1
Gross Unrealized Loss	1.4	0.5
Fixed Maturity Securities	2,161.6	2,216.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	22,935.7	22,696.6
Gross Unrealized Gain	2,731.0	4,245.3
Gross Unrealized Loss	193.9	31.5
Fixed Maturity Securities	25,472.8	26,910.4
Redeemable Preferred Stocks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	33.0	33.0
Gross Unrealized Gain	5.2	6.3
Gross Unrealized Loss	0	0
Fixed Maturity Securities	$ 38.2	$ 39.3

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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