Amortized Cost and Fair Values of Securities by Security Type (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 37,632.6	$ 37,751.5
Gross Unrealized Gain	6,850.2	7,277.5
Gross Unrealized Loss	46.3	56.0
Fixed Maturity Securities	44,436.5	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	1,027.3	1,020.9
Gross Unrealized Gain	309.2	329.0
Gross Unrealized Loss	1.4	1.1
Fixed Maturity Securities	1,335.1	1,348.8
States, Municipalities, and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,550.8	1,498.4
Gross Unrealized Gain	308.5	316.2
Gross Unrealized Loss	5.5	7.8
Fixed Maturity Securities	1,853.8	1,806.8
Foreign Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,197.3	1,280.4
Gross Unrealized Gain	260.5	226.6
Gross Unrealized Loss	0	0
Fixed Maturity Securities	1,457.8	1,507.0
Public Utility, Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	9,316.7	9,294.3
Gross Unrealized Gain	1,782.4	1,865.0
Gross Unrealized Loss	9.9	15.1
Fixed Maturity Securities	11,089.2	11,144.2
Mortgage/Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,767.1	1,927.9
Gross Unrealized Gain	275.6	289.1
Gross Unrealized Loss	0.5	0.5
Fixed Maturity Securities	2,042.2	2,216.5
All Other Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	22,740.4	22,696.6
Gross Unrealized Gain	3,907.7	4,245.3
Gross Unrealized Loss	29.0	31.5
Fixed Maturity Securities	26,619.1	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	6.3	6.3
Gross Unrealized Loss	0	0
Fixed Maturity Securities	$ 39.3	$ 39.3

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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-Publisher FASB

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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-Number FAS115-1/124-1

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

-Number 133

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+ Details

- Definition

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.

+ References+ Details

- 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.

+ References+ Details