Summary of Significant Accounting Principles (Policies)	9 Months Ended
Sep. 30, 2013
Principles of Financial Statement Presentation	*(a) Principles of Financial Statement Presentation* This report should be read in conjunction with the Annual Report on Form 10-K for the year ended December 31, 2012 (the “2012 10-K”) and the Quarterly Reports on Form 10-Q for the quarters ended March 31, 2013 and June 30, 2013 of Alleghany Corporation, a Delaware corporation (“Alleghany”). Unless the context otherwise requires, references to “Alleghany” include Alleghany together with its subsidiaries. Alleghany is engaged in the property and casualty reinsurance and insurance business. Reinsurance business is conducted through certain subsidiaries of Alleghany’s wholly-owned subsidiary Transatlantic Holdings, Inc. (“TransRe”), which was acquired in a merger transaction (the “merger”) on March 6, 2012 (the “TransRe Acquisition Date”). Insurance business is conducted through certain subsidiaries of Alleghany’s wholly-owned subsidiary, Alleghany Insurance Holdings LLC (“AIHL”). TransRe, through its principal wholly-owned subsidiaries, Transatlantic Reinsurance Company (“TRC”) and TransRe Zurich Ltd. (“TRZ”), offers reinsurance capacity to reinsurance and insurance companies for property and casualty products. These products are distributed through brokers and on a direct basis in both the domestic and foreign markets. TransRe is headquartered in New York, New York with six other locations in the United States and has operations worldwide, including: Africa, Australia, Bermuda, Canada, three locations in Asia, three locations in Central and South America, and seven locations in the United Kingdom and Europe. TRC is licensed, accredited or authorized or can serve as a reinsurer in all 50 states and the District of Columbia in the United States and in Puerto Rico and Guam. TRC is also licensed in Bermuda, Canada, Japan, the United Kingdom, the Dominican Republic, the Hong Kong Special Administrative Region of the People’s Republic of China, Germany and Australia. In addition, TRZ is licensed as a reinsurer in Switzerland. AIHL’s insurance business is conducted through its wholly-owned subsidiaries RSUI Group, Inc. (“RSUI”), Capitol Transamerica Corporation and Platte River Insurance Company (collectively, “CATA”), and Pacific Compensation Corporation (“PCC”). AIHL Re LLC (“AIHL Re”), a captive reinsurance subsidiary of AIHL, provides reinsurance to Alleghany operating units and affiliates. Alleghany also owns and manages properties in the Sacramento, California region through its subsidiary Alleghany Properties Holdings LLC (“Alleghany Properties”), and owns Stranded Oil Resources Corporation (“Stranded Oil”), an exploration and production company focused on enhanced oil recovery. On April 26, 2012, Alleghany’s majority-owned subsidiary BKH Holdings, Inc. acquired Bourn & Koch, Inc. (“BKI”), a manufacturer and remanufacturer/retrofitter of precision machine tools and supplier of replacement parts, headquartered in Rockford, Illinois. On August 30, 2013, Alleghany invested in R.C. Tway Company, LLC (“Kentucky Trailer”), a manufacturer of custom trailers and truck bodies for the moving and storage industry and other markets, headquartered in Louisville, Kentucky, for a controlling equity interest. Kentucky Trailer’s results have been included in Alleghany’s Consolidated Financial Statements beginning August 30, 2013. Alleghany also owns a minority stake in Homesite Group Incorporated (“Homesite”), a national, full-service, mono-line provider of homeowners insurance, which was purchased in December 2006 for cash consideration of $120.0 million. On September 4, 2013, American Family Insurance Company, a Wisconsin-based mutual insurance company, entered into a merger agreement with Homesite to purchase all of its issued and outstanding capital stock (including shares underlying options and restricted stock units) for cash consideration of $616.0 million, subject to adjustment for changes in Homesite stockholders’ equity between June 30, 2013 and the closing of the transaction and transaction expenses. Alleghany currently owns approximately 28 percent of the fully diluted shares of Homesite common stock. The transaction, which is subject to regulatory approvals and other closing conditions, is expected to close in the fourth quarter of 2013 or first quarter of 2014, at which time Alleghany expects to record a gain on its investment in Homesite. In addition to Homesite, Alleghany owns approximately 38 percent of ORX Exploration, Inc. (“ORX”), a regional oil and gas exploration and production company. The Homesite and ORX investments are reflected in Alleghany’s financial statements in other invested assets. Alleghany manages, sources, executes and monitors its private capital investments, which include Stranded Oil, BKI, Kentucky Trailer and ORX, primarily through its wholly-owned subsidiary, Alleghany Capital Corporation. Alleghany’s public equity investments, including those held by TransRe’s and AIHL’s operating units, are managed primarily by its indirect, wholly-owned subsidiary, Alleghany Capital Partners LLC. The financial statements contained in this Quarterly Report on Form 10-Q are unaudited, but reflect all adjustments that, in the opinion of management, are necessary for a fair statement of results of the interim periods covered thereby. All adjustments are of a normal and recurring nature except as described herein. The accompanying consolidated financial statements include the results of Alleghany and its wholly-owned and majority-owned or controlled subsidiaries and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). All significant inter-company balances and transactions have been eliminated in consolidation. The results of TransRe are included starting from the TransRe Acquisition Date, the results of BKI are included beginning April 26, 2012 and the results of Kentucky Trailer are included beginning August 30, 2013. The portion of stockholders’ equity, net earnings and accumulated other comprehensive income that is not attributable to Alleghany stockholders is presented on the Consolidated Balance Sheets and the Consolidated Statements of Earnings and Comprehensive Income as noncontrolling interest. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions. These estimates and assumptions affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Alleghany relies on historical experience and on various other assumptions that it believes to be reasonable under the circumstances to make judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those reported results to the extent that those estimates and assumptions prove to be inaccurate. Changes in estimates are reflected in the consolidated statement of earnings and comprehensive income in the period in which the change is made. The results of operations for any interim period are not necessarily indicative of results for the full year.
Other Significant Accounting Principles	*(b) Other Significant Accounting Principles* Alleghany’s significant accounting principles can be found in Note 1 to the Notes to Consolidated Financial Statements set forth in Part II, Item 8, of the 2012 10-K.
Recent Accounting Standards	*(c) Recent Accounting Standards* Recently Adopted In February 2013, the Financial Accounting Standards Board (the “FASB”) issued guidance on the amounts reclassified out of accumulated other comprehensive income. Other comprehensive income includes gains and losses that are initially excluded from net earnings for a given period, and later reclassified out of accumulated other comprehensive income into net earnings. This guidance requires companies to disclose information about reclassifications out of accumulated other comprehensive income in one place. This guidance is effective for interim and annual periods beginning after December 15, 2012. Alleghany adopted this guidance in the first quarter of 2013, and the implementation did not have an impact on its results of operations and financial condition. See Note 6(b). In December 2011, the FASB issued guidance on disclosure requirements related to offsetting arrangements. The guidance provides for additional financial statement disclosure regarding offsetting and related arrangements to enable financial statement users to understand the effect of those arrangements on an entity’s financial position. This guidance is effective for interim and annual reporting periods beginning on or after January 1, 2013. Alleghany adopted this guidance in the first quarter of 2013, and the implementation did not have a material impact on its results of operations and financial condition.

(a) Principles of Financial Statement Presentation

This report should be read in conjunction with the Annual Report on Form 10-K for the year ended December 31, 2012 (the “2012 10-K”) and the Quarterly Reports on Form 10-Q for the quarters ended March 31, 2013 and June 30, 2013 of Alleghany Corporation, a Delaware corporation (“Alleghany”). Unless the context otherwise requires, references to “Alleghany” include Alleghany together with its subsidiaries.

Alleghany is engaged in the property and casualty reinsurance and insurance business. Reinsurance business is conducted through certain subsidiaries of Alleghany’s wholly-owned subsidiary Transatlantic Holdings, Inc. (“TransRe”), which was acquired in a merger transaction (the “merger”) on March 6, 2012 (the “TransRe Acquisition Date”). Insurance business is conducted through certain subsidiaries of Alleghany’s wholly-owned subsidiary, Alleghany Insurance Holdings LLC (“AIHL”).

TransRe, through its principal wholly-owned subsidiaries, Transatlantic Reinsurance Company (“TRC”) and TransRe Zurich Ltd. (“TRZ”), offers reinsurance capacity to reinsurance and insurance companies for property and casualty products. These products are distributed through brokers and on a direct basis in both the domestic and foreign markets. TransRe is headquartered in New York, New York with six other locations in the United States and has operations worldwide, including: Africa, Australia, Bermuda, Canada, three locations in Asia, three locations in Central and South America, and seven locations in the United Kingdom and Europe. TRC is licensed, accredited or authorized or can serve as a reinsurer in all 50 states and the District of Columbia in the United States and in Puerto Rico and Guam. TRC is also licensed in Bermuda, Canada, Japan, the United Kingdom, the Dominican Republic, the Hong Kong Special Administrative Region of the People’s Republic of China, Germany and Australia. In addition, TRZ is licensed as a reinsurer in Switzerland.

AIHL’s insurance business is conducted through its wholly-owned subsidiaries RSUI Group, Inc. (“RSUI”), Capitol Transamerica Corporation and Platte River Insurance Company (collectively, “CATA”), and Pacific Compensation Corporation (“PCC”). AIHL Re LLC (“AIHL Re”), a captive reinsurance subsidiary of AIHL, provides reinsurance to Alleghany operating units and affiliates.

Alleghany also owns and manages properties in the Sacramento, California region through its subsidiary Alleghany Properties Holdings LLC (“Alleghany Properties”), and owns Stranded Oil Resources Corporation (“Stranded Oil”), an exploration and production company focused on enhanced oil recovery. On April 26, 2012, Alleghany’s majority-owned subsidiary BKH Holdings, Inc. acquired Bourn & Koch, Inc. (“BKI”), a manufacturer and remanufacturer/retrofitter of precision machine tools and supplier of replacement parts, headquartered in Rockford, Illinois. On August 30, 2013, Alleghany invested in R.C. Tway Company, LLC (“Kentucky Trailer”), a manufacturer of custom trailers and truck bodies for the moving and storage industry and other markets, headquartered in Louisville, Kentucky, for a controlling equity interest. Kentucky Trailer’s results have been included in Alleghany’s Consolidated Financial Statements beginning August 30, 2013.

Alleghany also owns a minority stake in Homesite Group Incorporated (“Homesite”), a national, full-service, mono-line provider of homeowners insurance, which was purchased in December 2006 for cash consideration of $120.0 million. On September 4, 2013, American Family Insurance Company, a Wisconsin-based mutual insurance company, entered into a merger agreement with Homesite to purchase all of its issued and outstanding capital stock (including shares underlying options and restricted stock units) for cash consideration of $616.0 million, subject to adjustment for changes in Homesite stockholders’ equity between June 30, 2013 and the closing of the transaction and transaction expenses. Alleghany currently owns approximately 28 percent of the fully diluted shares of Homesite common stock. The transaction, which is subject to regulatory approvals and other closing conditions, is expected to close in the fourth quarter of 2013 or first quarter of 2014, at which time Alleghany expects to record a gain on its investment in Homesite.

In addition to Homesite, Alleghany owns approximately 38 percent of ORX Exploration, Inc. (“ORX”), a regional oil and gas exploration and production company. The Homesite and ORX investments are reflected in Alleghany’s financial statements in other invested assets.

Alleghany manages, sources, executes and monitors its private capital investments, which include Stranded Oil, BKI, Kentucky Trailer and ORX, primarily through its wholly-owned subsidiary, Alleghany Capital Corporation. Alleghany’s public equity investments, including those held by TransRe’s and AIHL’s operating units, are managed primarily by its indirect, wholly-owned subsidiary, Alleghany Capital Partners LLC.

The financial statements contained in this Quarterly Report on Form 10-Q are unaudited, but reflect all adjustments that, in the opinion of management, are necessary for a fair statement of results of the interim periods covered thereby. All adjustments are of a normal and recurring nature except as described herein.

The accompanying consolidated financial statements include the results of Alleghany and its wholly-owned and majority-owned or controlled subsidiaries and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). All significant inter-company balances and transactions have been eliminated in consolidation. The results of TransRe are included starting from the TransRe Acquisition Date, the results of BKI are included beginning April 26, 2012 and the results of Kentucky Trailer are included beginning August 30, 2013.

The portion of stockholders’ equity, net earnings and accumulated other comprehensive income that is not attributable to Alleghany stockholders is presented on the Consolidated Balance Sheets and the Consolidated Statements of Earnings and Comprehensive Income as noncontrolling interest.

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions. These estimates and assumptions affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements, as well as the reported amounts of revenues and expenses during the reporting period. Alleghany relies on historical experience and on various other assumptions that it believes to be reasonable under the circumstances to make judgments about the carrying value of assets and liabilities that are not readily apparent from other sources. Actual results may differ materially from those reported results to the extent that those estimates and assumptions prove to be inaccurate. Changes in estimates are reflected in the consolidated statement of earnings and comprehensive income in the period in which the change is made. The results of operations for any interim period are not necessarily indicative of results for the full year.