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Available-for-Sale Securities (Detail) (USD $)
In Millions

Sep. 30, 2011

Dec. 31, 2010

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

$ 4,427.4

$ 4,352.9

Gross unrealized gains

217.1

275.4

Gross unrealized losses

(120.3)

(30.4)

4,524.2

4,597.9

Equity Securities | Common Stock

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

1,428.2

^[1]

1,310.0

^[1]

Gross unrealized gains

87.2

^[1]

196.3

^[1]

Gross unrealized losses

(110.0)

^[1]

(5.6)

^[1]

1,405.4

^[1]

1,500.7

^[1]

Debt Securities

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

2,822.6

2,778.1

Gross unrealized gains

129.9

79.1

Gross unrealized losses

(10.3)

(24.8)

2,942.2

2,832.4

Debt Securities | Mortgage and Asset Backed Securities

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

990.6

^[2]

841.0

^[2]

Gross unrealized gains

46.3

^[2]

31.8

^[2]

Gross unrealized losses

(4.5)

^[2]

(6.3)

^[2]

1,032.4

^[2]

866.5

^[2]

Debt Securities | U.S. Government obligations

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

292.3

334.4

Gross unrealized gains

7.2

4.6

Gross unrealized losses

(1.2)

299.5

337.8

Debt Securities | States, municipalities and political subdivision bonds

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

1,037.4

1,058.1

Gross unrealized gains

60.2

25.4

Gross unrealized losses

(1.0)

(15.0)

1,096.6

1,068.5

Debt Securities | Foreign bonds

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

92.0

112.7

Gross unrealized gains

2.3

2.4

Gross unrealized losses

(0.8)

(0.9)

93.5

114.2

Debt Securities | Corporate bonds and other

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

410.3

431.9

Gross unrealized gains

13.9

14.9

Gross unrealized losses

(4.0)

(1.4)

420.2

445.4

Short-term Investments

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

176.6

264.8

176.6

264.8

Alleghany Insurance Holdings, LLC

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

3,928.3

3,760.3

Gross unrealized gains

177.5

232.7

Gross unrealized losses

(120.3)

(30.4)

3,985.5

3,962.6

Corporate activities

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

499.1

592.6

Gross unrealized gains

39.6

42.7

538.7

635.3

Mortgage and Asset Backed Securities

Schedule of Available-for-sale Securities [Line Items]

Amortized Cost or Cost

$ 990.6

[1]

Of the $1,405.4 million of fair value as of September 30, 2011, $914.0 million related to certain energy sector businesses. Of the $1,500.7 million of fair value as of December 31, 2010, $1,004.8 million related to certain energy sector businesses.

[2]

Of the $1,032.4 million of fair value as of September 30, 2011, $538.3 million related to residential mortgage-backed securities ("RMBS") $213.1 million related to commercial mortgage-backed securities ("CMBS") and $281.0 million related to other asset-backed securities. Of the $866.5 million of fair value as of December 31, 2010, $499.9 million related to RMBS, $173.4 million related to CMBS and $193.2 million related to other asset-backed securities.

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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