Alleghany (Y) 10-Q 5 May 20 2020 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	Apr. 26, 2020
Cover [Abstract]
Amendment Flag	false
Document Type	10-Q
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Registrant Name	ALLEGHANY CORP /DE
Entity Central Index Key	0000775368
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		14,309,682
Entity Shell Company	false
Entity Small Business	false
Entity Emerging Growth Company	false
Trading Symbol	Y
Title of 12(b) Security	Common Stock
Security Exchange Name	NYSE
Entity File Number	1-9371
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	51-0283071
Entity Address, Address Line One	1411 Broadway, 34th Floor
Entity Address, City or Town	NY
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10018
City Area Code	212
Local Phone Number	752-1356
Document Quarterly Report	true
Document Transition Report	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019
Investments:
Equity securities	$ 1,710,193		$ 2,505,496
Debt securities	13,337,754	[1]	14,211,745
Short-term investments	536,105		914,776
Marketable Securities, Total	15,584,052		17,632,017
Commercial mortgage loans	682,655		686,206
Other invested assets	541,512		573,605
Total investments	16,808,219		18,891,828
Cash	1,555,185		1,179,098
Accrued investment income	94,301		96,516
Premium balances receivable	1,091,980		948,010
Reinsurance recoverables	1,628,649		1,681,962
Ceded unearned premiums	266,173		248,153
Deferred acquisition costs	541,040		522,577
Property and equipment at cost, net of accumulated depreciation and amortization	208,300		205,397
Goodwill	530,228		523,021
Intangible assets, net of amortization	688,233		685,953
Current taxes receivable	30,791		4,081
Net deferred tax assets	189,814		5,860
Funds held under reinsurance agreements	779,272		755,515
Other assets	1,340,756		1,183,633
Total assets	25,752,941		26,931,604
Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity
Loss and loss adjustment expenses	12,066,438		11,928,359
Unearned premiums	2,668,409		2,566,170
Senior notes and other debt	1,514,381		1,751,113
Reinsurance payable	184,779		188,399
Other liabilities	1,253,511		1,516,076
Total liabilities	17,687,518		17,950,117
Redeemable noncontrolling interests	208,361		204,753
Common stock	17,460		17,460
Contributed capital	3,610,137		3,608,638
Accumulated other comprehensive income (loss)	(129,343)		171,350
Treasury stock	(1,496,554)		(1,455,877)
Retained earnings	5,855,362		6,435,163
Total stockholders’ equity attributable to Alleghany stockholders	7,857,062		8,776,734
Total liabilities, redeemable noncontrolling interests and stockholders’ equity	$ 25,752,941		$ 26,931,604
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Equity securities, cost		$ 1,358,265	$ 1,625,256
Debt securities, amortized cost		13,320,808	$ 13,798,576
Allowance for credit losses	[1]	$ 31,375
Common stock, Shares authorized		22,000,000	22,000,000
Common stock, Shares issued		17,459,961	17,459,961
Treasury stock, shares		3,150,279	3,095,333
[1]	See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.

Consolidated Statements of Earn

Consolidated Statements of Earnings and Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues
Net premiums earned	$ 1,444,420	$ 1,297,309
Net investment income	111,982	123,049
Change in the fair value of equity securities	(522,976)	392,353
Net realized capital gains	13,026	4,442
Credit losses for available for sale securities	(31,375)
Credit losses for available for sale securities		(10,020)
Noninsurance revenue	467,849	514,123
Total revenues	1,482,926	2,321,256
Costs and Expenses
Net loss and loss adjustment expenses	1,027,444	792,498
Commissions, brokerage and other underwriting expenses	441,396	420,126
Other operating expenses	466,661	502,418
Corporate administration	(14,307)	19,369
Amortization of intangible assets	9,568	6,900
Interest expense	18,356	23,531
Total costs and expenses	1,949,118	1,764,842
(Losses) earnings before income taxes	(466,192)	556,414
Income taxes	(104,720)	108,512
Net (losses) earnings	(361,472)	447,902
Net (losses) earnings attributable to noncontrolling interest	(254)	7,675
Net (losses) earnings attributable to Alleghany stockholders	(361,218)	440,227
Net (losses) earnings	(361,472)	447,902
Other comprehensive income (loss):
Change in unrealized gains, net of deferred taxes	(276,629)	183,387
Less: reclassification for net realized capital gains and credit losses for available for sale securities, net of taxes	(10,848)	4,407
Change in unrealized currency translation adjustment, net of deferred taxes	(13,733)	(885)
Retirement plans	517	937
Comprehensive (loss) income	(662,165)	635,748
Comprehensive (loss) income attributable to noncontrolling interests	(254)	7,675
Comprehensive (loss) income attributable to Alleghany stockholders	$ (661,911)	$ 628,073
Basic (losses) earnings per share attributable to Alleghany stockholders	$ (25.19)	$ 30.40
Diluted (losses) earnings per share attributable to Alleghany stockholders	$ (25.37)	$ 30.39

Consolidated Statements of Ea_2

Consolidated Statements of Earnings and Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Change in unrealized gains, deferred taxes	$ (73,534)	$ 48,749
Reclassification for net realized capital gains and credit losses for available for sale securities, taxes	(2,883)	1,171
Change in unrealized currency translation adjustment, deferred taxes	$ (3,651)	$ (235)

Consolidated Statements of Chan

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Contributed Capital	Accumulated Other Comprehensive Income (loss)	Treasury Stock	Retained Earnings
Beginning Balance at Dec. 31, 2018	$ 7,692,710	$ 17,460	$ 3,612,830	$ (202,003)	$ (1,312,939)	$ 5,577,362
Net (losses) earnings	440,227					440,227
Other comprehensive income (loss), net of tax:
Retirement plans	937			937
Change in unrealized appreciation of investments, net	187,794			187,794
Change in unrealized currency translation adjustment, net	(885)			(885)
Comprehensive (loss) income attributable to Alleghany stockholders	628,073			187,846		440,227
Treasury stock repurchase	(80,486)				(80,486)
Other, net	(3,810)		(4,176)		366
Ending Balance at Mar. 31, 2019	8,236,487	17,460	3,608,654	(14,157)	(1,393,059)	6,017,589
Redeemable noncontrolling interests balance at Dec. 31, 2018	169,762
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	7,675
Comprehensive (loss) income attributable to noncontrolling interests	7,675
Other net changes to redeemable noncontrolling interests	(1,716)
Redeemable noncontrolling interests balance at Mar. 31, 2019	175,721
Beginning Balance at Dec. 31, 2019	8,776,734	17,460	3,608,638	171,350	(1,455,877)	6,435,163
Cumulative effect of adoption of new accounting pronouncements at Dec. 31, 2019	(3,570)					(3,570)
Net (losses) earnings	(361,218)					(361,218)
Other comprehensive income (loss), net of tax:
Retirement plans	517			517
Change in unrealized appreciation of investments, net	(287,477)			(287,477)
Change in unrealized currency translation adjustment, net	(13,733)			(13,733)
Comprehensive (loss) income attributable to Alleghany stockholders	(661,911)			(300,693)		(361,218)
Dividends paid	(215,013)					(215,013)
Treasury stock repurchase	(44,268)				(44,268)
Other, net	5,090		1,499		3,591
Ending Balance at Mar. 31, 2020	7,857,062	$ 17,460	$ 3,610,137	$ (129,343)	$ (1,496,554)	$ 5,855,362
Redeemable noncontrolling interests balance at Dec. 31, 2019	204,753
Redeemable Non-controlling Interests
Net (losses) earnings attributable to redeemable noncontrolling interests	(254)
Comprehensive (loss) income attributable to noncontrolling interests	(254)
Other net changes to redeemable noncontrolling interests	3,862
Redeemable noncontrolling interests balance at Mar. 31, 2020	$ 208,361

Consolidated Statements of Ch_2

Consolidated Statements of Changes in Stockholders' Equity (Parenthetical) - shares	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Statement Of Stockholders Equity [Abstract]
Common stock, Shares issued	17,459,961	17,459,961	17,459,961	17,459,961
Treasury stock, shares	3,150,279	3,095,333	3,012,224	2,883,452

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities
Net (losses) earnings	$ (361,472)	$ 447,902
Adjustments to reconcile net earnings to net cash provided by (used in) operating activities:
Depreciation and amortization	33,158	27,732
Change in the fair value of equity securities	522,976	(392,353)
Net realized capital (gains) losses	(13,026)	(4,442)
Credit losses for available for sale securities	31,375
Credit losses for available for sale securities		10,020
(Increase) decrease in reinsurance recoverables, net of reinsurance payable	49,693	165,434
(Increase) decrease in premium balances receivable	(143,970)	(103,538)
(Increase) decrease in ceded unearned premiums	(18,020)	(9,849)
(Increase) decrease in deferred acquisition costs	(18,463)	(23,537)
(Increase) decrease in funds held under reinsurance agreements	(23,757)	19,984
Increase (decrease) in unearned premiums	102,239	101,816
Increase (decrease) in loss and loss adjustment expenses	138,079	(395,170)
Change in unrealized foreign currency exchange rate losses (gains)	80,530	1,070
Other, net	(386,627)	88,546
Net adjustments	354,187	(514,287)
Net cash provided by (used in) operating activities	(7,285)	(66,385)
Cash flows from investing activities
Purchases of debt securities	(1,993,449)	(2,460,366)
Purchases of equity securities	(331,453)	(69,729)
Sales of debt securities	1,960,691	987,132
Maturities and redemptions of debt securities	459,655	235,521
Sales of equity securities	602,627	1,510,222
Net (purchases) sales of short-term investments	378,854	(376,414)
Net (purchases) sales and maturities of commercial mortgage loans	3,551	29
(Purchases) sales of property and equipment	(8,840)	(12,956)
Purchases of affiliates and subsidiaries, net of cash acquired	(26,383)	1,157
Other, net	(152,416)	73,423
Net cash provided by (used in) investing activities	892,837	(111,981)
Cash flows from financing activities
Repayment of senior notes	(307,095)
Treasury stock acquisitions	(44,268)	(80,486)
Increase (decrease) in other debt	62,659	8,178
Cash dividends paid	(215,013)
Other, net	3,664	(11,421)
Net cash provided by (used in) financing activities	(500,053)	(83,729)
Effect of foreign exchange rate changes on cash	(9,412)	1,396
Net increase (decrease) in cash	376,087	(260,699)
Cash at beginning of period	1,179,098	1,038,763
Cash at end of period	1,555,185	778,064
Supplemental disclosures of cash flow information
Interest paid	17,065	19,368
Income taxes paid (refund received)	$ 24,604	$ (39,657)

Summary of Significant Accounti

Summary of Significant Accounting Principles	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Principles	1. Summary of Significant Accounting Principles (a) Principles of Financial Statement Presentation This Quarterly Report on Form 10-Q (this “Form 10-Q”) should be read in conjunction with the Annual Report on Form 10-K for the year ended December 31, 2019 (the “2019 Form 10-K”) of Alleghany Corporation (“Alleghany”). Alleghany Corporation, a Delaware corporation, owns and supports certain operating subsidiaries and manages investments, anchored by a core position in property and casualty reinsurance and insurance. Through its wholly-owned subsidiary Transatlantic Holdings, Inc. (“TransRe”), an Alleghany subsidiary since March 2012, Alleghany is engaged in the property and casualty reinsurance business. Through its wholly-owned subsidiary Alleghany Insurance Holdings LLC (“AIHL”), Alleghany is engaged in the property and casualty insurance business. AIHL’s insurance operations are principally conducted by its subsidiaries RSUI Group, Inc. (“RSUI”) and CapSpecialty, Inc. (“CapSpecialty”). RSUI and CapSpecialty have been subsidiaries of AIHL since July 2003 and January 2002, respectively. AIHL Re LLC (“AIHL Re”), a captive reinsurance company, which provides reinsurance to Alleghany’s current and former insurance operating subsidiaries and affiliates, has been a subsidiary of Alleghany since its formation in May 2006. Although Alleghany’s primary sources of revenues and earnings are its reinsurance and insurance operations and investments, Alleghany also generates revenues and expenses from a diverse portfolio of middle market businesses that are owned and managed through its wholly-owned subsidiary Alleghany Capital Corporation (“Alleghany Capital”). Alleghany Capital’s businesses include: • Precision Cutting Technologies, Inc. (“PCT”), a holding company headquartered in Rockford, Illinois, with four operating businesses: (i) Bourn & Koch, Inc., a provider of precision automated machine tool solutions; (ii) Diamond Technology Innovations, Inc., a manufacturer of waterjet orifices and nozzles and a provider of related services; (iii) Coastal Industrial Distributors, LLC, a provider of high-performance solid carbide end mills; and (iv) as of March 2020, Supermill LLC, a manufacturer of high-performance carbide end mills; • 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; • IPS-Integrated Project Services, LLC (“IPS”), a design, engineering, procurement, construction management and validation service provider focused on the global pharmaceutical and biotechnology industries, headquartered in Blue Bell, Pennsylvania; • Jazwares, LLC (together with its affiliates, “Jazwares”), a global toy, entertainment and musical instrument company, headquartered in Sunrise, Florida; • WWSC Holdings, LLC (“W&W|AFCO Steel”), a structural steel fabricator and erector, headquartered in Oklahoma City, Oklahoma; • CHECO Holdings, LLC (“Concord”), a hotel management and development company, headquartered in Raleigh, North Carolina; and • as of March 31, 2020, In addition, Alleghany owns certain other holding-company investments. Alleghany’s wholly-owned subsidiary Stranded Oil Resources Corporation (“SORC”) is an exploration and production company focused on enhanced oil recovery, headquartered in Golden, Colorado. Alleghany’s wholly-owned subsidiary Alleghany Properties Holdings LLC (“Alleghany Properties”) owns and manages certain properties in the Sacramento, California region. Alleghany’s public equity investments are managed primarily through Alleghany’s wholly-owned subsidiary Roundwood Asset Management LLC. Unless the context otherwise requires, references to “Alleghany” include Alleghany together with its subsidiaries. The accompanying consolidated financial statements include the results of Alleghany and its wholly-owned and majority-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). All material inter-company balances and transactions have been eliminated in consolidation. The portion of stockholders’ equity, net earnings and comprehensive income that is not attributable to Alleghany stockholders is presented on the consolidated balance sheets, the consolidated statements of earnings and comprehensive income and the consolidated statements of changes in stockholders’ equity as noncontrolling interests. Because all noncontrolling interests have the option to sell their ownership interests to Alleghany in the future (generally through 2024), the portion of stockholders’ equity that is not attributable to Alleghany stockholders is presented on the consolidated balance sheets and the consolidated statements of changes in stockholders’ equity as redeemable noncontrolling interests for all periods presented. In addition, Alleghany accretes the redeemable noncontrolling interests up to their future estimated redemption value over the period from the date of issuance to the earliest redemption date. The redemption value of the equity interests is generally based on the subsidiary’s earnings in specified periods preceding the applicable redemption date, calculated based on either a specified formula or an independent fair market valuation. During the first three months of 2020, the noncontrolling interests outstanding were approximately as follows: Kentucky Trailer - 23 percent; IPS - 15 percent; Jazwares - 25 percent; W&W|AFCO Steel - 20 percent; and Concord - 15 percent. Prior to April 1, 2019, the noncontrolling interests of PCT were approximately 11 percent. All noncontrolling interest holders of PCT have exercised their repurchase options and sold their ownership interests to PCT effective April 1, 2019. 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 as of 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 and reported revenues and expenses 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 statements of earnings and comprehensive income in the period in which the changes are made. (b) Other Significant Accounting Principles Alleghany’s significant accounting principles can be found in Note 1 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K. (c) Recent Accounting Standards Recently Adopted - 2019 In February 2016, the Financial Accounting Standards Board (the “FASB”) issued guidance on leases. Under this guidance, a lessee is required to recognize lease liabilities and corresponding right-of-use assets for leases with terms of more than one year, whereas under the prior guidance, a lessee was only required to recognize assets and liabilities for those leases qualifying as capital leases. This guidance also requires new disclosures about the amount, timing and uncertainty of cash flows arising from leases. The accounting by lessors remains largely unchanged. This guidance became effective in the first quarter of 2019 for public companies, with early adoption permitted. A modified retrospective transition approach was elected for all leases in existence as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements. Alleghany adopted this guidance in the first quarter of 2019 and the implementation did not have a material impact on its results of operations and financial condition. As part of its implementation, Alleghany elected to not separate lease components from non-lease components (such as office cleanings, security and maintenance services provided by Alleghany’s lessors for certain of its leases). Alleghany also elected the package of practical expedients under the transition guidance, which allowed Alleghany to not reevaluate existing lease classifications, among others. As of January 1, 2019, Alleghany’s adoption of this guidance resulted in recognition of an additional right-of-use asset of approximately $0.2 billion and a corresponding lease liability of $0.2 billion. See Note 12(b) to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for further information on Alleghany’s leases. In August 2017, the FASB issued guidance that simplifies the requirements to achieve hedge accounting, better reflects the economic results of hedging in financial statements and improves the alignment between hedge accounting and a company’s risk management activities. This guidance was effective in the first quarter of 2019 for public companies, with early adoption permitted. Alleghany adopted this guidance in the first quarter of 2019 and the implementation did not have a material impact on its results of operations and financial condition. Recently Adopted – 2020 In June 2016, the FASB issued guidance on credit losses. Under this guidance, a company is required to measure all expected credit losses on loans, reinsurance recoverables and other financial assets accounted for at cost or amortized cost, as applicable, over the remaining expected life of such assets. Estimates of expected credit losses are to be based on historical experience, current conditions and reasonable and supportable forecasts. Under former FASB guidance, credit losses on these assets generally required a company to recognize credit losses when it was probable that a loss had been incurred. Credit losses for securities accounted for on an available for sale (“AFS”) basis are to be measured in a manner similar to GAAP as applied under former guidance and cannot exceed the amount by which the fair value is less than the amortized cost, although the new guidance removes the length of time a security has been in an unrealized loss position as a possible indication of a credit impairment. Credit losses for all financial assets are to be recorded through an allowance for credit losses. Subsequent reversals in credit loss estimates are permitted and are to be recognized in earnings. This guidance also requires new disclosures about the significant estimates and judgments used in estimating credit losses, as well as the credit quality of financial assets. This guidance was effective in the first quarter of 2020 for public companies , with early adoption permitted. Alleghany adopt ed this guidance in the first quarter of 2020. As of January 1, 2020, Alleghany increased its allowances for credit losses on certain financial assets accounted for at cost or amortized cost by $4.5 million and recorded an after-tax cumulative effect reduction in opening retained earnings of $3.6 million. The increase in allowances for credit losses primarily relates to reinsurance recoverables and commercial mortgage loans. See Note 4 of this Form 10-Q for further information on Alleghany’s reinsurance recoverables and Note 3(i) of this Form 10-Q for further information on Alleghany’s investments in commercial mortgage loans. The increase in allowances for credit losses also relates to premium balances receivable. Estimates of expected credit losses for all of these assets are based on certain market-based indicators of credit quality, which are updated on an annual basis and re-assessed at least quarterly. Related credit loss expenses are recorded as a component of other operating expenses. As of January 1, 2020, credit losses for Alleghany’s AFS securities are recorded through an allowance for credit losses on the consolidated balance sheets and as credit losses for AFS securities in the consolidated statements of earnings and comprehensive income. See Note 3(f) of this Form 10-Q for further information on the credit quality for Alleghany’s AFS securities. In January 2017, the FASB issued guidance that simplifies the subsequent measurement of goodwill. Under this guidance, if an initial qualitative assessment indicates that the fair value of an operating subsidiary may be less than its carrying amount, an impairment charge is recognized for the amount by which the carrying amount of the operating subsidiary exceeds its estimated fair value. Any resulting impairment loss recognized cannot exceed the total amount of goodwill associated with the operating subsidiary. This guidance was effective in the first quarter of 2020 for public companies, with early adoption permitted. Alleghany adopted this guidance in the first quarter of 2020 and the implementation did not have a material impact on its results of operations and financial condition. See Note 2 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for further information on Alleghany’s goodwill. In August 2018, the FASB issued guidance that changes the financial statement disclosure requirements for measuring fair value. With respect to financial instruments classified as “Level 3” in the fair value disclosure hierarchy, the guidance requires certain additional disclosures for public companies related to amounts included in other comprehensive income and significant unobservable inputs used in the valuation, while removing disclosure requirements related to an entity’s overall valuation processes. The guidance also removes certain disclosure requirements related to transfers between financial instruments classified as “Level 1” and “Level 2” and provides clarification on certain other existing disclosure requirements. This guidance was effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted with respect to any eliminated or modified disclosures. Alleghany adopted this guidance in the first quarter of 2020 and the implementation did not impact its results of operations or financial condition. See Note 2 of this Form 10-Q for further information on the fair value of Alleghany’s financial instruments. In March 2020, the FASB issued guidance to expedite and simplify the accounting associated with the anticipated migration away from the widely-used London Inter-bank Offered Rate and other similar rates as benchmark interest rates after 2021. Under pre-existing GAAP, such modifications made to: (i) loans and certain other contracts would require re-assessments of the accounting for those contacts, such as whether they were extinguished and remeasured from an accounting perspective; and (ii) derivative contracts may cause a change in accounting, such as a possible dedesignation of hedge accounting. This new guidance largely eliminates these requirements as a result of this migration to one or more new benchmark rates and is generally applicable for contract modifications made prior to December 31, 2022. Alleghany adopted this guidance in March 2020 and the implementation did not have an impact on its results of operations and financial condition.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	2. Fair Value of Financial Instruments The following table presents the carrying value and estimated fair value of Alleghany’s consolidated financial instruments as of March 31, 2020 and December 31, 2019: March 31, 2020 December 31, 2019 Carrying Value Fair Value Carrying Value Fair Value ($ in millions) Assets Investments (excluding equity method investments and loans) (1) $ 15,584.6 $ 15,584.6 $ 17,632.3 $ 17,632.3 Liabilities Senior notes and other debt (2) $ 1,514.4 $ 1,674.8 $ 1,751.1 $ 1,962.7 (1) This table includes debt and equity securities, as well as partnership and non-marketable equity investments accounted for at fair value that are included in other invested assets. This table excludes investments accounted for using the equity method and commercial mortgage loans that are accounted for at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is disclosed below. (2) See Note 11 of this Form 10-Q for additional information on the senior notes and other debt. The following tables present Alleghany’s financial instruments measured at fair value and the level of the fair value hierarchy of inputs used as of March 31, 2020 and December 31, 2019: Level 1 Level 2 Level 3 Total ($ in millions) As of March 31, 2020 Equity securities: Common stock $ 1,702.6 $ 3.5 $ — $ 1,706.1 Preferred stock — 2.1 2.0 4.1 Total equity securities 1,702.6 5.6 2.0 1,710.2 Debt securities: U.S. Government obligations — 846.3 — 846.3 Municipal bonds — 2,272.6 — 2,272.6 Foreign government obligations — 696.0 — 696.0 U.S. corporate bonds — 2,805.2 589.5 3,394.7 Foreign corporate bonds — 1,125.9 164.7 1,290.6 Mortgage and asset-backed securities: Residential mortgage-backed securities (“RMBS”) (1) — 1,697.4 2.5 1,699.9 Commercial mortgage-backed securities (“CMBS”) — 838.1 4.5 842.6 Other asset-backed securities (2) — 1,548.4 746.7 2,295.1 Total debt securities (3) — 11,829.9 1,507.9 13,337.8 Short-term investments — 536.1 — 536.1 Other invested assets (4) — — 0.5 0.5 Total investments (excluding equity method investments and loans) $ 1,702.6 $ 12,371.6 $ 1,510.4 $ 15,584.6 Senior notes and other debt $ — $ 1,245.0 $ 429.8 $ 1,674.8 Level 1 Level 2 Level 3 Total ($ in millions) As of December 31, 2019 Equity securities: Common stock $ 2,498.1 $ 3.3 $ — $ 2,501.4 Preferred stock — 2.1 2.0 4.1 Total equity securities 2,498.1 5.4 2.0 2,505.5 Debt securities: U.S. Government obligations — 1,215.0 — 1,215.0 Municipal bonds — 2,307.9 — 2,307.9 Foreign government obligations — 690.7 — 690.7 U.S. corporate bonds — 2,754.0 605.0 3,359.0 Foreign corporate bonds — 1,208.7 168.7 1,377.4 Mortgage and asset-backed securities: RMBS (1) — 1,838.5 1.9 1,840.4 CMBS — 865.9 5.8 871.7 Other asset-backed securities (2) — 1,692.9 856.7 2,549.6 Total debt securities — 12,573.6 1,638.1 14,211.7 Short-term investments — 914.8 — 914.8 Other invested assets (4) — — 0.3 0.3 Total investments (excluding equity method investments and loans) $ 2,498.1 $ 13,493.8 $ 1,640.4 $ 17,632.3 Senior notes and other debt $ — $ 1,595.6 $ 367.1 $ 1,962.7 (1) Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS. (2) Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively. (3) Includes allowance for credit losses as of March 31, 2020. See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities. ( 4 ) Includes partnership and non-marketable equity investments accounted for at fair value, and excludes investments accounted for using the equity method. In the three months ended March 31, 2020, Alleghany transferred out of Level 3 $14.7 million of financial instruments, principally due to an increase in observable inputs related to the valuation of such assets. Specifically, during the first three months of 2020, there was a decrease in the weight given to non-binding broker quotes and, as a result, there was a corresponding increase in quoted prices for similar assets in active markets. Of the $14.7 million of transfers, $11.4 million related to other asset-backed securities, with the remainder related to foreign corporate bonds. In the three months ended March 31, 2019, Alleghany transferred into Level 3 $14.7 million of foreign corporate bonds, principally due to a decrease in observable inputs related to the valuation of such assets. Specifically, during the first three months of 2019, there was an increase in the weight given to non-binding broker quotes and, as a result, there was a corresponding decrease in quoted prices for similar assets in active markets. In the three months ended March 31, 2019, Alleghany transferred out of Level 3 $1.0 million of U.S. corporate bonds, principally due to an increase in observable inputs related to the valuation of such assets. Specifically, during the first three months of 2019, there was a decrease in the weight given to non-binding broker quotes, and as a result, there was a corresponding increase in quoted prices for similar assets in active markets. The following tables present reconciliations of the changes in Level 3 assets during the three months ended March 31, 2020 and 2019 measured at fair value: Debt Securities Mortgage and asset-backed Three Months Ended March 31, 2020 Preferred Stock U.S. Corporate Bonds Foreign Corporate Bonds RMBS CMBS Other Asset- backed Securities Other Invested Assets (1) Total ($ in millions) Balance as of January 1, 2020 $ 2.0 $ 605.0 $ 168.7 $ 1.9 $ 5.8 $ 856.7 $ 0.3 $ 1,640.4 Net realized/unrealized gains (losses) included in: Net earnings (losses) (2) — 0.2 — — — (0.1 ) — 0.1 Other comprehensive income (loss) — (18.8 ) (7.3 ) (0.2 ) (1.3 ) (101.4 ) 0.2 (128.8 ) Purchases — 6.7 7.7 0.9 — 43.6 — 58.9 Sales — — — — — (8.3 ) — (8.3 ) Issuances — — — — — — — — Settlements — (3.6 ) (1.1 ) (0.1 ) — (32.4 ) — (37.2 ) Transfers into Level 3 — — — — — — — — Transfers out of Level 3 — — (3.3 ) — — (11.4 ) — (14.7 ) Balance as of March 31, 2020 $ 2.0 $ 589.5 $ 164.7 $ 2.5 $ 4.5 $ 746.7 $ 0.5 $ 1,510.4 Debt Securities Mortgage and asset-backed Three Months Ended March 31, 2019 Preferred Stock U.S. Corporate Bonds Foreign Corporate Bonds CMBS Other Asset- backed Securities Other Invested Assets (1) Total ($ in millions) Balance as of January 1, 2019 $ 5.4 $ 425.7 $ 126.9 $ — $ 1,266.9 $ 0.7 $ 1,825.6 Net realized/unrealized gains (losses) included in: Net (losses) (2) — (9.9 ) — — — — (9.9 ) Other comprehensive income (loss) (0.8 ) 15.7 2.6 — 15.2 (0.2 ) 32.5 Purchases — 5.8 13.8 0.2 49.6 — 69.4 Sales — — (5.6 ) — (116.9 ) — (122.5 ) Issuances — — — — — — — Settlements — (2.6 ) (1.0 ) — (35.8 ) — (39.4 ) Transfers into Level 3 — — 14.7 — — — 14.7 Transfers out of Level 3 — (1.0 ) — — — — (1.0 ) Balance as of March 31, 2019 $ 4.6 $ 433.7 $ 151.4 $ 0.2 $ 1,179.0 $ 0.5 $ 1,769.4 (1) Includes partnership and non-marketable equity investments accounted for at fair value. (2) There were no credit losses recorded in net earnings related to Level 3 assets still held as of March 31, 2020 and 2019. The in crease in senior n otes and other debt included in Level 3 for the first three months of 20 20 primarily reflects in creased borrowings at Jazwares due to an i n crease in working capital needs. Although Alleghany is responsible for the determination of the fair value of Alleghany’s financial assets and the supporting methodologies and assumptions, it employs third-party valuation service providers to gather, analyze and interpret market information and derive fair values based upon relevant methodologies and assumptions for individual instruments. When those providers are unable to obtain sufficient market observable information upon which to estimate the fair value for a particular security, fair value is determined either by requesting a quote, which is generally non-binding, from brokers who are knowledgeable about these securities or by employing widely accepted valuation models. As of March 31, 2020 and December 31, 2019, the fair value for the vast majority of debt securities included in Level 3 was provided by such third-party valuation service providers, and as such, valuation details on these securities are generally not available to Alleghany. Alleghany employs specific control processes to determine the reasonableness of the fair values of its financial assets and liabilities. Alleghany’s processes are designed to ensure that the values received or internally estimated are accurately recorded and that the data inputs and the valuation techniques used are appropriate, consistently applied and that the assumptions are reasonable and consistent with the objective of determining fair value. Alleghany assesses the reasonableness of individual security values received from valuation service providers through various analytical techniques. In addition, Alleghany validates the reasonableness of fair values by comparing information obtained from Alleghany’s valuation service providers to other third-party valuation sources for selected securities. Alleghany also validates prices obtained from brokers for selected securities through reviews by those who have relevant expertise and who are independent of those charged with executing investing transactions. See Note 1(c) to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for Alleghany’s accounting policy on fair value.

Investments

Investments	3 Months Ended
	Mar. 31, 2020
Investments [Abstract]
Investments	3. Investments (a) Unrealized Gains and Losses The following tables present the amortized cost and the fair value of AFS securities as of March 31, 2020 and December 31, 2019: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Allowance for Credit Losses (2) Fair Value ($ in millions) As of March 31, 2020 Debt securities: U.S. Government obligations $ 793.5 $ 53.0 $ (0.2 ) $ — $ 846.3 Municipal bonds 2,158.3 117.2 (2.9 ) — 2,272.6 Foreign government obligations 669.1 27.6 (0.7 ) — 696.0 U.S. corporate bonds 3,386.8 103.5 (69.1 ) (26.5 ) 3,394.7 Foreign corporate bonds 1,311.2 14.5 (32.7 ) (2.4 ) 1,290.6 Mortgage and asset-backed securities: RMBS 1,613.7 86.9 (0.2 ) (0.5 ) 1,699.9 CMBS 870.6 15.9 (43.9 ) — 842.6 Other asset-backed securities (1) 2,517.6 5.6 (226.1 ) (2.0 ) 2,295.1 Total debt securities 13,320.8 424.2 (375.8 ) (31.4 ) 13,337.8 Short-term investments 536.1 — — — 536.1 Total investments $ 13,856.9 $ 424.2 $ (375.8 ) $ (31.4 ) $ 13,873.9 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value ($ in millions) As of December 31, 2019 Debt securities: U.S. Government obligations $ 1,198.5 $ 19.3 $ (2.8 ) $ 1,215.0 Municipal bonds 2,190.5 118.7 (1.3 ) 2,307.9 Foreign government obligations 675.9 16.4 (1.6 ) 690.7 U.S. corporate bonds 3,206.2 155.1 (2.3 ) 3,359.0 Foreign corporate bonds 1,348.1 32.2 (2.9 ) 1,377.4 Mortgage and asset-backed securities: RMBS 1,785.1 56.1 (0.8 ) 1,840.4 CMBS 849.9 23.2 (1.4 ) 871.7 Other asset-backed securities (1) 2,544.5 24.8 (19.7 ) 2,549.6 Total debt securities 13,798.7 445.8 (32.8 ) 14,211.7 Short-term investments 914.8 — — 914.8 Total investments $ 14,713.5 $ 445.8 $ (32.8 ) $ 15,126.5 (1) Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively. (2) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. (b) Contractual Maturity The following table presents the amortized cost and estimated fair value of debt securities by contractual maturity as of March 31, 2020. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. Amortized Cost Fair Value ($ in millions) As of March 31, 2020 Short-term investments due in one year or less $ 536.1 $ 536.1 Mortgage and asset-backed securities (1) 5,001.9 4,837.6 Debt securities with maturity dates: One year or less 432.8 432.3 Over one through five years 3,174.0 3,201.3 Over five through ten years 2,728.0 2,770.8 Over ten years 1,984.1 2,095.8 Total debt securities $ 13,320.8 $ 13,337.8 (1) Mortgage and asset-backed securities by their nature do not generally have single maturity dates. (c) Net Investment Income The following table presents net investment income for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Interest income $ 119.3 $ 119.2 Dividend income 10.1 9.8 Investment expenses (7.1 ) (7.5 ) Partnerships and other investment results (10.3 ) 1.5 Total $ 112.0 $ 123.0 As of March 31, 2020, non-income producing invested assets were immaterial. (d) Change in the Fair Value of Equity Securities The following table presents changes in the fair value of equity securities for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Change in the fair value of equity securities sold during the period $ (164.9 ) $ 62.1 Change in the fair value of equity securities held at the end of the period (358.1 ) 330.3 Change in the fair value of equity securities $ (523.0 ) $ 392.4 (e) Realized Gains and Losses The proceeds from sales of debt and equity securities were $2.6 billion and $2.5 billion for the three months ended March 31, 2020 and 2019, respectively. Realized capital gains and losses for the three months ended March 31, 2020 and 2019 primarily reflect the sale of debt securities and, for the three months ended March 31, 2020, a $30.0 million impairment charge from a write-down of certain SORC assets arising from a significant decline in energy prices as of March 31, 2020. Realized capital gains and losses for the three months ended March 31, 2020 also include a $5.0 million realized gain from a reduction of certain contingent consideration liabilities at the PCT-level in connection with its acquisition of a provider of high-performance solid carbide end mills in June 2019, and a $7.1 million realized loss as a result of an early redemption of debt (see Note 11(a) of this Form 10-Q for further information on this early redemption). The following table presents amounts of gross realized capital gains and gross realized capital losses for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Gross realized capital gains $ 59.0 $ 12.1 Gross realized capital losses (46.0 ) (7.7 ) Net realized capital gains $ 13.0 $ 4.4 Gross realized capital losses exclude credit losses for AFS securities, as discussed below. (f) Credit quality for AFS securities Alleghany holds its debt securities as AFS and, as such, these securities are recorded at fair value. Credit losses for AFS securities are recorded through an allowance for credit losses. Changes in the allowance for credit losses are recorded for (or as a reversal of) credit losses for AFS securities. Any portion of a decline in fair value related to a debt security that is believed to arise from factors other than credit is recorded as a component of other comprehensive income rather than charged against earnings. Alleghany continually monitors the difference between amortized cost and the estimated fair value of its debt investments. The analysis of a security’s decline in value is performed in its functional currency. Debt securities in an unrealized loss position are evaluated for credit losses if they meet any of the following criteria: (i) they are trading at a discount of at least 20 percent to amortized cost and have a credit rating below investment grade or are not rated; (ii) there has been a negative credit or news event with respect to the issuer that could indicate the existence of a credit loss; or (iii) Alleghany intends to sell, or it is more likely than not that Alleghany will sell, the debt security before recovery of its amortized cost basis. If Alleghany intends to sell, or it is more likely than not that Alleghany will sell, a debt security before recovery of its amortized cost basis, the total amount of the unrealized loss position is recognized as a credit loss in earnings. To the extent that a debt security that is in an unrealized loss position is not impaired based on the preceding, Alleghany will consider a debt security to be impaired when it believes it to be probable that Alleghany will not be able to collect the entire amortized cost basis. For debt securities in an unrealized loss position as of the end of each quarter, Alleghany develops a best estimate of the present value of expected cash flows. If the results of the cash flow analysis indicate that Alleghany will not recover the full amount of its amortized cost basis in the debt security, Alleghany records a credit loss in earnings equal to the difference between the present value of expected cash flows and the amortized cost basis of the debt security. If applicable, the difference between the total unrealized loss position on the debt security and the total loss recognized in earnings is the non-credit related portion, which is recorded as a component of other comprehensive income. In developing the cash flow analyses for debt securities, Alleghany considers various factors for the different categories of debt securities. For municipal bonds, Alleghany takes into account the taxing power of the issuer, source of revenue, credit risk and enhancements and pre-refunding. For mortgage and asset-backed securities, Alleghany discounts its best estimate of future cash flows at an effective rate equal to the original effective yield of the security or, in the case of floating rate securities, at the current coupon. Alleghany’s models include assumptions about prepayment speeds, default and delinquency rates, underlying collateral (if any), credit ratings, credit enhancements and other observable market data. For corporate bonds, Alleghany reviews business prospects, credit ratings and available information from asset managers and rating agencies for individual securities. Credit losses for AFS securities in the first three months of 2020 reflect $ 31.4 Credit losses for AFS securities in the first three months of 2019 reflect $10.0 million of unrealized losses on debt securities, primarily related to the energy sector and the deterioration of creditworthiness of the issuers. The following table presents a rollforward of Alleghany’s allowance for credit losses for AFS securities for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) — Provision for credit losses 31.4 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 31.4 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. The gross unrealized investment losses for debt securities as of March 31, 2020 were deemed to be temporary, based on, among other factors: (i) the relative magnitude to which the fair value of these investments had been below cost were not indicative of a credit loss; (ii) the absence of compelling evidence that would cause Alleghany to call into question the financial condition or near-term business prospects of the issuer of the security; and (iii) Alleghany’s ability and intent to hold the security for a period of time sufficient to allow for any anticipated recovery. Alleghany’s methodology for assessing credit losses contains inherent risks and uncertainties which could include, but are not limited to, incorrect assumptions about financial condition, liquidity or future prospects, inadequacy of any underlying collateral and unfavorable changes in economic conditions or social trends, interest rates or credit ratings. Alleghany’s consolidated investment portfolio consists mainly of highly rated and liquid debt and equity securities listed on national securities exchanges. The overall credit quality of the debt securities portfolio is measured using the lowest rating of three large, reputable rating agencies. In this regard, the overall weighted-average credit quality rating of Alleghany’s debt securities portfolio as of March 31 , 20 20 and December 31, 201 9 , was AA-. Although a portion of Alleghany’s debt securities, which consist predomi nantly of municipal bonds, is insured by third-party financial guaranty insurance companies, the impact of such insurance was not significant to the debt securities ’ credit quality rating as of March 31 , 20 20 . The following table presents the ratings of Alleghany’s debt securities as of March 31, 2020 : Ratings as of March 31, 2020 AAA / Aaa AA / Aa A BBB / Baa Below BBB / Baa or Not Rated (1) Total ($ in millions) U.S. Government obligations $ — $ 846.3 $ — $ — $ — $ 846.3 Municipal bonds 184.2 1,526.5 489.4 71.6 0.9 2,272.6 Foreign government obligations 445.4 153.8 95.8 1.0 — 696.0 U.S. corporate bonds 9.0 158.0 1,586.4 1,298.2 343.1 3,394.7 Foreign corporate bonds 225.2 95.9 607.4 325.9 36.2 1,290.6 Mortgage and asset-backed securities: RMBS 2.5 1,692.3 — 1.0 4.1 1,699.9 CMBS 283.6 400.8 157.8 0.4 — 842.6 Other asset-backed securities 961.8 513.6 457.9 274.6 87.2 2,295.1 Total debt securities $ 2,111.7 $ 5,387.2 $ 3,394.7 $ 1,972.7 $ 471.5 $ 13,337.8 Percentage of debt securities, before allowance for credit losses 15.8 % 40.4 % 25.5 % 14.8 % 3.5 % 100.0 % (1) Consists of $109.3 million of securities rated BB / Ba, $187.5 million of securities rated B, $26.2 million of securities rated CCC, $0.4 million of securities rated CC, $2.1 million of securities rated below CC and $146.0 million of not rated securities. (g) Aging of Gross Unrealized Losses The following tables present gross unrealized losses and related fair values for Alleghany’s AFS securities for which an allowance for credit losses has not been recorded, grouped by duration of time in a continuous unrealized loss position, as of March 31, 2020 and December 31, 2019: Less Than 12 Months 12 Months or More Total Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses ($ in millions) As of March 31, 2020 Debt securities: U.S. Government obligations $ 8.7 $ 0.2 $ — $ — $ 8.7 $ 0.2 Municipal bonds 129.4 2.9 0.1 — 129.5 2.9 Foreign government obligations 76.5 0.7 — — 76.5 0.7 U.S. corporate bonds 1,023.5 68.5 3.6 0.6 1,027.1 69.1 Foreign corporate bonds 656.5 32.6 17.4 0.1 673.9 32.7 Mortgage and asset-backed securities: RMBS 15.2 0.2 0.4 — 15.6 0.2 CMBS 436.4 43.9 0.2 — 436.6 43.9 Other asset-backed securities 1,529.0 139.5 491.3 86.6 2,020.3 226.1 Total temporarily impaired securities $ 3,875.2 $ 288.5 $ 513.0 $ 87.3 $ 4,388.2 $ 375.8 Less Than 12 Months 12 Months or More Total Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses ($ in millions) As of December 31, 2019 Debt securities: U.S. Government obligations $ 120.3 $ 2.0 $ 99.7 $ 0.8 $ 220.0 $ 2.8 Municipal bonds 90.2 1.3 4.3 — 94.5 1.3 Foreign government obligations 198.5 1.2 33.8 0.4 232.3 1.6 U.S. corporate bonds 113.8 1.6 27.6 0.7 141.4 2.3 Foreign corporate bonds 212.0 2.3 66.9 0.6 278.9 2.9 Mortgage and asset-backed securities: RMBS 48.7 0.3 48.6 0.5 97.3 0.8 CMBS 126.8 1.4 1.2 — 128.0 1.4 Other asset-backed securities 422.9 2.7 715.9 17.0 1,138.8 19.7 Total temporarily impaired securities $ 1,333.2 $ 12.8 $ 998.0 $ 20.0 $ 2,331.2 $ 32.8 As of March 31, 2020, Alleghany held a total of 1,672 debt securities that were in an unrealized loss position, of which 126 securities were in an unrealized loss position continuously for 12 months or more. The unrealized losses associated with these debt securities consisted of losses related primarily to other asset-backed securities, U.S. corporate bonds and CMBS. (h) Investments in Variable Interest Entities In December 2012, TransRe obtained an ownership interest in Pillar Capital Holdings Limited (“Pillar Holdings”), a Bermuda- based insurance asset manager focused on collateralized reinsurance and catastrophe insurance-linked securities. Additionally, TransRe invested $175.0 million and AIHL invested $25.0 million in limited partnership funds managed by Pillar Holdings (the “Funds”). The objective of the Funds is to create portfolios with attractive risk-reward characteristics and low correlation with other asset classes, using the extensive reinsurance and capital market experience of the principals of Pillar Holdings. Alleghany has concluded that both Pillar Holdings and the Funds (collectively, the “Pillar Investments”) represent variable interest entities and that Alleghany is not the primary beneficiary, as it does not have the ability to direct the activities that most significantly impact each entity’s economic performance. Therefore, the Pillar Investments are not consolidated and are accounted for under the equity method of accounting. Alleghany’s potential maximum loss in the Pillar Investments is limited to its cumulative net investment. As of March 31, 2020, Alleghany’s carrying value in the Pillar Investments, as determined under the equity method of accounting, was $194.7 million, which is reported as a component of other invested assets and is net of returns of capital received from the Pillar Investments. (i) Investments in Commercial Mortgage Loans As of March 31, 2020, the carrying value of Alleghany’s commercial mortgage loan portfolio was $682.7 million, representing the unpaid principal balance on the loans, less allowance for credit losses. The estimated fair value of the commercial mortgage loan portfolio approximated carrying value as of March 31, 2020 and December 31, 2019. The commercial mortgage loan portfolio consists primarily of first mortgages on commercial properties in major metropolitan areas in the U.S. The loans earn interest at fixed- and floating-rates, and mature in two to ten years from loan origination. As of March 31, 2020, the vast majority of loans in Alleghany’s portfolio were originated in the 2016 through 2019 years. The principal amounts of the loans were no more than approximately two-thirds of the property’s appraised value at the time the loans were made and the estimated fair value of underlying collateral was approximately double that of the commercial mortgage loan portfolio carrying value as of March 31, 2020. Fair value estimates of underlying collateral are updated on a rolling basis annually, with a portion of the portfolio updated each quarter. As of March 31, 2020, there was no loan in default or in arrears. The following table presents a rollforward of Alleghany’s allowance for credit losses on commercial mortgage loans for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) 0.8 Provision for credit losses (2) 1.2 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 2.0 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. (2) Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

Reinsurance Ceded

Reinsurance Ceded	3 Months Ended
	Mar. 31, 2020
Insurance [Abstract]
Reinsurance Ceded	4. Reinsurance Ceded (a) Overview Alleghany’s reinsurance and insurance subsidiaries reinsure portions of the risks they underwrite in order to reduce the effect of individual or aggregate exposure to losses, manage capacity, protect capital resources, reduce volatility in specific lines of business, improve risk-adjusted portfolio returns and enable them to increase gross premium writings and risk capacity without requiring additional capital. Alleghany’s reinsurance and insurance subsidiaries generally purchase reinsurance and retrocessional coverages from highly- rated third-party reinsurers or on a collateralized basis. If the assuming reinsurers are unable or unwilling to meet the obligations assumed under the applicable reinsurance agreements, Alleghany’s reinsurance and insurance subsidiaries would remain liable for such reinsurance portion not paid by these reinsurers. As such, funds, trust agreements and letters of credit are held to collateralize a portion of Alleghany’s reinsurance recoverables and Alleghany’s reinsurance and insurance subsidiaries reinsure portions of the risks they underwrite or assume with multiple reinsurance programs. A summary of the more significant programs follows. TransRe enters into various retrocession arrangements, including property catastrophe retrocession contracts, to manage the effects of individual or aggregate exposure to losses, reduce volatility in specific lines of business, improve risk-adjusted portfolio returns, strengthen its market position and enhance capital efficiency. These include excess-of-loss and quota share treaties in traditional rated form as well as catastrophe bonds and other collateralized insurance-linked structures. TransRe’s retrocession protections generally have a one-year four-year RSUI reinsures its property lines of business through a program consisting of surplus share treaties, facultative placements, and per risk and catastrophe excess of loss treaties. RSUI’s catastrophe reinsurance program and property per risk reinsurance program each run on an annual basis from May 1 to the following April 30 and portions expired on April 30, 2020. Both programs were renewed on May 1, 2020 with substantially similar terms as the expired programs. (b) Reinsurance Recoverables Amounts recoverable from reinsurers are recognized in a manner consistent with the loss and loss adjustment expense (“LAE”) liabilities associated with the reinsurance placement and presented on the balance sheet as reinsurance recoverables, and are recorded after an allowance for credit losses. Reinsurance recoverable balances as of March 31, 2020 and December 31, 2019 are presented in the table below: As of March 31, As of December 31, 2020 2019 ($ in millions) Reinsurance recoverables on paid losses $ 75.9 $ 98.1 Ceded outstanding loss and LAE 1,559.5 1,583.9 Reinsurance recoverables, before allowance for credit losses 1,635.4 1,682.0 Allowance for credit losses (6.8 ) — Total $ 1,628.6 $ 1,682.0 The following table presents information regarding concentration of Alleghany’s reinsurance recoverables and the ratings profile of our reinsurers as of March 31, 2020: Reinsurer (1) Rating (2) Amount Percentage ($ in millions) Syndicates at Lloyd's of London A (Excellent) $ 132.9 8.1 % Kane SAC Ltd, Rondout Segregated Account (3) not rated 121.8 7.4 % PartnerRe Ltd A (Excellent) 97.4 6.0 % Fairfax Financial Holdings Ltd A (Superior) 90.4 5.5 % Swiss Reinsurance Company A+ (Superior) 88.1 5.4 % Third Point Reinsurance Group A- (Excellent) 87.0 5.3 % Kane SAC Ltd, Bowery Segregated Account (3) not rated 77.4 4.7 % RenaissanceRe Holdings Ltd A+ (Superior) 75.4 4.6 % W.R. Berkley Corporation A+ (Superior) 67.7 4.1 % Liberty Mutual A (Excellent) 55.9 3.4 % All other reinsurers 741.4 45.5 % Total reinsurance recoverables, before allowance for credit losses (4) $ 1,635.4 100.0 % Allowance for credit losses (6.8 ) Total $ 1,628.6 Secured reinsurance recoverables (3) $ 659.6 40.3 % (1) Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company. (2) Represents the A.M. Best Company, Inc. financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due. (3) Represents reinsurance recoverables secured by funds held, trust agreements or letters of credit. (4) Approximately 66 percent of our reinsurance recoverables balance as of March 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit. The following table presents a rollforward of Alleghany’s allowance for credit losses on reinsurance recoverables for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) 3.3 Provision for credit losses (2) 3.5 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 6.8 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. (2) Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

Liability for Loss and LAE

Liability for Loss and LAE	3 Months Ended
	Mar. 31, 2020
Insurance [Abstract]
Liability for Loss and LAE	5. Liability for Loss and LAE (a) Liability Rollforward The following table presents the activity in the liability for loss and LAE for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Reserves as of January 1 $ 11,928.4 $ 12,250.3 Less: reinsurance recoverables (1) 1,583.9 1,857.4 Net reserves as of January 1 10,344.5 10,392.9 Other adjustments — (0.2 ) Incurred loss and LAE, net of reinsurance, related to: Current year 1,086.4 829.9 Prior years (59.0 ) (37.4 ) Total incurred loss and LAE, net of reinsurance 1,027.4 792.5 Paid loss and LAE, net of reinsurance, related to: (2) Current year 87.3 75.3 Prior years 770.2 914.8 Total paid loss and LAE, net of reinsurance 857.5 990.1 Foreign currency exchange rate effect (7.5 ) 17.2 Net reserves as of March 31 10,506.9 10,212.3 Reinsurance recoverables as of March 31 (1) 1,559.5 1,642.8 Reserves as of March 31 $ 12,066.4 $ 11,855.1 (1) Reinsurance recoverables in this table include only ceded loss and LAE reserves. (2) Includes paid losses, net of reinsurance, related to commutations. Gross loss and LAE reserves as of March 31, 2020 increased from December 31, 2019, primarily reflecting the impact of growing net premiums earned and catastrophe losses incurred in the first three months of 2020, partially offset by payments on catastrophe losses incurred in 2017, 2018 and 2019 and favorable prior accident year loss reserve development. (b) Liability Development The following table presents the (favorable) unfavorable prior accident year loss reserve development for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Reinsurance Segment Property: Catastrophe events $ (32.3 ) (1) $ 8.4 (2) Non-catastrophe (14.4 ) (3) (19.1 ) (4) Total (46.7 ) (10.7 ) Casualty & other: Malpractice Treaties (5) — — Other (11.0 ) (6) (25.7 ) (7) Total (11.0 ) (25.7 ) Total Reinsurance Segment (57.7 ) (36.4 ) Insurance Segment RSUI: Casualty (1.0 ) (8) (5.6 ) (9) Property and other (0.5 ) (10) 1.9 (11) Total (1.5 ) (3.7 ) CapSpecialty 0.2 (12) 2.7 (12) Total incurred related to prior years $ (59.0 ) $ (37.4 ) (1) Primarily reflects favorable prior accident year loss reserve development related to Typhoon Hagibis in the 2019 accident year and catastrophes in the 2018 accident year. (2) Primarily reflects unfavorable prior accident year loss reserve development related to Typhoon Jebi in the 2018 accident year, partially offset by favorable prior accident year loss reserve development related to the wildfires in California in the 2018 accident year. (3) Primarily reflects favorable prior accident year loss reserve development in the 2017 and 2019 accident years. (4) Primarily reflects favorable prior accident year loss reserve development in the 2016 through 2018 accident years. (5) Represents certain malpractice treaties pursuant to which the increased underwriting profits created by the favorable prior accident year loss reserve development are largely retained by the cedants. As a result, the favorable prior accident year loss reserve development is largely offset by an increase in profit commission expense incurred when such favorable prior accident year loss reserve development occurs. (6) Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and earlier accident years, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2014 through 2019 accident years. (7) Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business in the 2012 through 2017 prior accident years and in the professional liability lines of business in the 2007 and prior accident years. ( 8 ) Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years. ( 9 ) Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in the 2017 accident year. (1 0 ) Primarily reflects favorable prior accident year loss reserve development related to catastrophes in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to catastrophes in the 2017 accident year. (1 1 ) Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Harvey in the 2017 accident year and Florence in the 2018 accident year. (1 2 ) Primarily reflects unfavorable prior accident year loss reserve development in certain specialty lines of business written through a program administrator in connection with a terminated program in the 2009 and 2010 accident years.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	6. Income Taxes The Coronavirus Aid, Relief and Economic Security Act (the "CARES Act") was signed into law on March 27, 2020. Among other provisions, the CARES Act delayed certain employer payroll tax remittance deadlines and created or expanded certain income tax credits and loss carryback provisions and, as a result, Alleghany recorded a $6.9 million tax benefit as of March 31, 2020. The effective tax rate on earnings before income taxes for the first three months of 20 20 was 22.4 percent compared with percent for the first three months of 201 9 . The effective tax rate in the first three months of 2020 was calculated based on actual results through March 31, 2020 because management was not able to reliably estimate the annual effective tax rate in light of the significant losses incurred. The increase in the effective tax rate in the first quarter of 2020 from the first quarter of 2019 primarily reflects the impact of the CARES Act . Alleghany believes that, as of March 31, 2020, it had no material uncertain tax positions. Interest and penalties related to unrecognized tax expenses (benefits) are recognized in income tax expense, when applicable. There were no material liabilities for interest or penalties accrued as of March 31, 2020.

Stockholders' Equity

Stockholders' Equity	3 Months Ended
	Mar. 31, 2020
Stockholders Equity Note [Abstract]
Stockholders' Equity	7. Stockholders’ Equity (a) Common Stock Repurchases In June 2018, the Alleghany Board of Directors authorized the repurchase of shares of common stock of Alleghany, par value $1.00 per share (“Common Stock”), at such times and at prices as management determines to be advisable, up to an aggregate of $400.0 million. In September 2019, the Alleghany Board of Directors authorized, upon the completion of the program authorized in 2018, the repurchase of additional shares of Common Stock, at such times and at prices as management determines to be advisable, up to an aggregate of $500.0 million. As of March 31, 2020, Alleghany had $582.9 million remaining under its share repurchase authorizations. The following table presents the shares of Common Stock that Alleghany repurchased in the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 Shares repurchased 62,540 129,567 Cost of shares repurchased (in millions) $ 44.3 $ 80.5 Average price per share repurchased $ 707.84 $ 621.19 (b) Accumulated Other Comprehensive Income (Loss) The following tables present a reconciliation of the changes during the three months ended March 31, 2020 and 2019 in accumulated other comprehensive income (loss) attributable to Alleghany stockholders: Unrealized Appreciation of Investments Unrealized Currency Translation Adjustment Retirement Plans Total ($ in millions) Balance as of January 1, 2020 $ 321.0 $ (121.8 ) $ (27.9 ) $ 171.3 Other comprehensive income (loss), net of tax: Other comprehensive income (loss) before reclassifications (276.6 ) (13.7 ) 0.5 (289.8 ) Reclassifications from accumulated other comprehensive income (10.9 ) — — (10.9 ) Total (287.5 ) (13.7 ) 0.5 (300.7 ) Balance as of March 31, 2020 $ 33.5 $ (135.5 ) $ (27.4 ) $ (129.4 ) Unrealized Appreciation of Investments Unrealized Currency Translation Adjustment Retirement Plans Total ($ in millions) Balance as of January 1, 2019 $ (61.6 ) $ (124.7 ) $ (15.7 ) $ (202.0 ) Other comprehensive income (loss), net of tax: Other comprehensive income (loss) before reclassifications 183.4 (0.9 ) 0.9 183.4 Reclassifications from accumulated other comprehensive income 4.4 — — 4.4 Total 187.8 (0.9 ) 0.9 187.8 Balance as of March 31, 2019 $ 126.2 $ (125.6 ) $ (14.8 ) $ (14.2 ) The following table presents reclassifications out of accumulated other comprehensive income (loss) attributable to Alleghany stockholders during the three months ended March 31, 2020 and 2019: Accumulated Other Three Months Ended March 31, Comprehensive Income Component Line in Consolidated Statement of Earnings 2020 2019 ($ in millions) Unrealized appreciation of investments: Net realized capital gains (1) $ (45.1 ) $ (4.4 ) Credit losses for available for sale securities 31.4 10.0 Income taxes 2.8 (1.2 ) Total reclassifications: Net (earnings) losses $ (10.9 ) $ 4.4 (1) For the three months ended March 31, 2020, excludes a $7.1 million realized loss as a result of an early redemption of debt (see Note 11(a) of this Form 10-Q for further information), a $30.0 million impairment charge from a write-down of certain SORC assets (c) Special Dividend In February 2020, the Alleghany Board of Directors declared a special dividend of $15.00 per share for stockholders of record on March 5, 2020. On March 15, 2020, Alleghany paid dividends to stockholders totaling $215.0 million.

Earnings Per Share of Common St

Earnings Per Share of Common Stock	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Earnings Per Share of Common Stock	8. Earnings Per Share of Common Stock The following table presents a reconciliation of the earnings and share data used in the basic and diluted earnings per share computations for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions, except share amounts) Net earnings available to Alleghany stockholders $ (361.2 ) $ 440.2 Effect of dilutive securities (3.0 ) — Income available to common stockholders for diluted earnings per share $ (364.2 ) $ 440.2 Weighted average common shares outstanding applicable to basic earnings per share 14,340,669 14,481,255 Effect of dilutive securities 15,439 3,981 Adjusted weighted average common shares outstanding applicable to diluted earnings per share 14,356,108 14,485,236 Contingently issuable shares (1) 18,703 39,325 (1) Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	9. Commitments and Contingencies (a) General The ongoing COVID-19 global pandemic (the “Pandemic”) has significantly disrupted many aspects of society as well as financial markets, and has caused widespread global economic dislocation. Alleghany cannot reasonably estimate the duration or severity of the Pandemic, or the extent to which the related disruption may adversely impact its results of operations, financial position and cash flows, or those of its subsidiaries. Adverse impacts from the Pandemic in future periods may include realized and unrealized losses in Alleghany’s investment portfolio and receivables, as well as impairment losses on certain subsidiary goodwill and intangible assets. Certain of Alleghany’s subsidiaries are parties to pending litigation and claims in connection with the ordinary course of their businesses. Each such subsidiary makes provisions for estimated losses to be incurred in such litigation and claims, including legal costs. In the opinion of management, such provisions are adequate, and management does not believe that any pending litigation will have a material adverse effect on Alleghany’s consolidated results of operations, financial position or cash flows. (b) Leases Alleghany and its subsidiaries lease certain facilities, land, furniture and equipment under long-term, non-cancelable lease agreements that expire at various dates in future years. Most of Alleghany’s leases relate to office facilities. Alleghany’s lease agreements do not contain any material restrictive covenants and substantially all are considered to be operating leases. Additional information about leases can be found in Note 12(b) to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K.

Segments of Business

Segments of Business	3 Months Ended
	Mar. 31, 2020
Segment Reporting [Abstract]
Segments of Business	10. Segments of Business (a) Overview Alleghany’s segments are reported in a manner consistent with the way management evaluates the businesses. As such, Alleghany classifies its businesses into three reportable segments – reinsurance, insurance and Alleghany Capital. Reinsurance and insurance underwriting activities are evaluated separately from investment and other activities. Segment accounting policies are described in Note 1 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K. The reinsurance segment consists of property and casualty reinsurance operations conducted by TransRe’s reinsurance operating subsidiaries and is further reported through two major product lines – property and casualty & other. TransRe provides property and casualty reinsurance to insurers and other reinsurers through brokers and on a direct basis to ceding companies. TransRe writes a modest amount of property and casualty insurance business, which is included in the reinsurance segment. A significant portion of the premiums earned by TransRe’s operations are generated by offices located in Canada, Europe, Asia, Australia, Africa and those serving Latin America and the Caribbean. Although the majority of the premiums earned by these offices typically relate to the regions where they are located, a significant portion may be derived from other regions of the world, including the U.S. In addition, although a significant portion of the assets and liabilities of these foreign offices generally relate to the countries where the ceding companies and reinsurers are located, most investments are located in the country of domicile of these offices. The insurance segment consists of property and casualty insurance operations conducted in the U.S. by AIHL through its insurance operating subsidiaries RSUI and CapSpecialty. RSUI also writes a modest amount of assumed reinsurance business, which is included in the insurance segment. The Alleghany Capital segment consists of industrial operations, non-industrial operations and corporate operations at the Alleghany Capital level. Industrial operations are conducted through PCT, Kentucky Trailer, W&W|AFCO Steel and, through March 31, 2020, a 45 percent equity interest in Wilbert. Non-industrial operations are conducted through IPS, Jazwares and Concord. Corporate activities are not classified as a segment. The primary components of corporate activities are Alleghany Properties, SORC and activities at the Alleghany parent company. Corporate activities also include the elimination of minor activity between segments. In addition, corporate activities include interest expense associated with the senior notes issued by Alleghany, whereas interest expense associated with senior notes issued by TransRe is included in “Total Segments” and interest expense associated with other debt is included in Alleghany Capital. Information related to the senior notes and other debt can be found in Note 11 of this Form 10-Q. (b) Results The following tables present segment results for Alleghany’s three reportable segments and for corporate activities for the three months ended March 31, 2020 and 2019: Reinsurance Segment Insurance Segment Three Months Ended March 31, 2020 Property Casualty & other (1) Total RSUI Cap Specialty Total Subtotal Alleghany Capital Total Segments Corporate Activities Consolidated ($ in millions) Gross premiums written $ 412.6 $ 914.9 $ 1,327.5 $ 365.9 $ 85.5 $ 451.4 $ 1,778.9 $ — $ 1,778.9 $ (8.3 ) $ 1,770.6 Net premiums written 312.9 898.1 1,211.0 244.5 78.2 322.7 1,533.7 — 1,533.7 — 1,533.7 Net premiums earned 321.4 811.7 1,133.1 229.4 81.9 311.3 1,444.4 — 1,444.4 — 1,444.4 Net loss and LAE 247.9 614.3 862.2 116.0 49.2 165.2 1,027.4 — 1,027.4 — 1,027.4 Commissions, brokerage and other underwriting expenses 105.4 244.4 349.8 57.8 33.8 91.6 441.4 — 441.4 — 441.4 Underwriting (loss) profit (2) $ (31.9 ) $ (47.0 ) $ (78.9 ) $ 55.6 $ (1.1 ) $ 54.5 (24.4 ) — (24.4 ) — (24.4 ) Net investment income 109.7 1.8 111.5 0.5 112.0 Change in the fair value of equity securities (521.6 ) — (521.6 ) (1.4 ) (523.0 ) Net realized capital gains 45.1 4.9 50.0 (37.0 ) 13.0 Credit losses for available for sale securities (30.7 ) — (30.7 ) (0.7 ) (31.4 ) Noninsurance revenue 7.4 457.3 464.7 3.2 467.9 Other operating expenses 9.9 450.7 460.6 6.1 466.7 Corporate administration (2.0 ) — (2.0 ) (12.3 ) (14.3 ) Amortization of intangible assets 0.4 9.2 9.6 — 9.6 Interest expense 6.7 3.8 10.5 7.8 18.3 Earnings (losses) before income taxes $ (429.5 ) $ 0.3 $ (429.2 ) $ (37.0 ) $ (466.2 ) Reinsurance Segment Insurance Segment Three Months Ended March 31, 2019 Property Casualty & other (1) Total RSUI Cap Specialty Total Subtotal Alleghany Capital Total Segments Corporate Activities Consolidated ($ in millions) Gross premiums written $ 415.1 $ 816.5 $ 1,231.6 $ 299.0 $ 82.8 $ 381.8 $ 1,613.4 $ — $ 1,613.4 $ (6.8 ) $ 1,606.6 Net premiums written 316.2 791.4 1,107.6 201.4 76.7 278.1 1,385.7 — 1,385.7 — 1,385.7 Net premiums earned 308.9 720.3 1,029.2 192.4 75.7 268.1 1,297.3 — 1,297.3 — 1,297.3 Net loss and LAE 162.6 490.6 653.2 94.3 45.0 139.3 792.5 — 792.5 — 792.5 Commissions, brokerage and other underwriting expenses 103.7 232.3 336.0 53.5 30.6 84.1 420.1 — 420.1 — 420.1 Underwriting (loss) profit (2) $ 42.6 $ (2.6 ) $ 40.0 $ 44.6 $ 0.1 $ 44.7 84.7 — 84.7 — 84.7 Net investment income 117.9 1.3 119.2 3.8 123.0 Change in the fair value of equity securities 389.1 — 389.1 3.3 392.4 Net realized capital gains 4.1 0.3 4.4 — 4.4 Credit losses for available for sale securities (10.0 ) — (10.0 ) — (10.0 ) Noninsurance revenue 5.4 505.0 510.4 3.7 514.1 Other operating expenses 29.7 465.4 495.1 7.3 502.4 Corporate administration 1.1 — 1.1 18.3 19.4 Amortization of intangible assets 0.2 6.7 6.9 — 6.9 Interest expense 6.7 4.3 11.0 12.5 23.5 Earnings (losses) before income taxes $ 553.5 $ 30.2 $ 583.7 $ (27.3 ) $ 556.4 (1) Primarily consists of the following reinsurance lines of business: directors’ and officers’ liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; mortgage reinsurance; surety; and credit. (2) Underwriting profit represents net premiums earned less net loss and LAE and commissions, brokerage and other underwriting expenses, all as determined in accordance with GAAP, and does not include net investment income, change in the fair value of equity securities, net realized capital gains, credit losses for available for sale securities, noninsurance revenue, other operating expenses, corporate administration, amortization of intangible assets or interest expense. Underwriting profit does not replace earnings before income taxes determined in accordance with GAAP as a measure of profitability. Rather, Alleghany believes that underwriting profit enhances the understanding of its reinsurance and insurance segments’ operating results by highlighting net earnings attributable to their underwriting performance. Earnings before income taxes (a GAAP measure) may show a profit despite an underlying underwriting loss. Where underwriting losses persist over extended periods, a reinsurance or an insurance company’s ability to continue as an ongoing concern may be at risk. Therefore, Alleghany views underwriting profit as an important measure in the overall evaluation of performance. (c) Identifiable Assets and Equity The following table presents identifiable assets, the portion of identifiable assets related to cash and invested assets and equity attributable to Alleghany for Alleghany’s reportable segments and for corporate activities as of March 31, 2020: Identifiable Assets Invested Assets and Cash Equity Attributable to Alleghany ($ in millions) Reinsurance segment $ 16,819.8 $ 13,077.2 $ 4,774.9 Insurance segment 6,414.1 4,697.6 2,377.5 Subtotal 23,233.9 17,774.8 7,152.4 Alleghany Capital 2,131.7 271.5 1,000.7 Total segments 25,365.6 18,046.3 8,153.1 Corporate activities 387.4 317.2 (296.0 ) Consolidated $ 25,753.0 $ 18,363.5 $ 7,857.1 (d) Alleghany Capital Noninsurance Revenue For Alleghany Capital’s industrial and non-industrial operations, noninsurance revenue consists of the sale of manufactured goods and services. The following table presents noninsurance revenue for the Alleghany Capital segment for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Industrial (1) $ 242.5 $ 260.1 Non-Industrial (2) 214.7 244.9 Corporate & other 0.1 — Alleghany Capital $ 457.3 $ 505.0 (1) For the three months ended March 31, 2020 and 2019, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time. (2) For the three months ended March 31, 2020 and 2019, approximately 82 percent 66 percent, respectively, of noninsurance revenue was recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.

Debt

Debt	3 Months Ended
	Mar. 31, 2020
Debt Disclosure [Abstract]
Debt	11. Debt (a) Senior Notes On September 20, 2010, Alleghany completed a public offering of $300.0 million aggregate principal amount of its 5.625% senior notes due on September 15, 2020 (the “2020 Senior Notes”). The 2020 Senior Notes were unsecured and unsubordinated general obligations of Alleghany. Interest on the 2020 Senior Notes was payable semi-annually on March 15 and September 15 of each year. The terms of the 2020 Senior Notes permitted redemption prior to their maturity. The indenture under which the 2020 Senior Notes were issued contains covenants that impose conditions on Alleghany’s ability to create liens on, or engage in sales of, the capital stock of AIHL or RSUI. The 2020 Senior Notes were issued at approximately 99.6 percent of par, resulting in proceeds after underwriting discount, commissions and other expenses of $298.9 million and an effective yield of approximately 5.67 percent. On January 15, 2020, Alleghany redeemed all of its outstanding 2020 Senior Notes for $312.7 million, consisting of the $300.0 million aggregate principal amount redeemed, $7.1 million of redemption premium and $5.6 million of accrued and unpaid interest on the principal amount being redeemed to the date of redemption. As a result of this early redemption of the 2020 Senior Notes, Alleghany recorded a realized loss, before tax, of $7.1 million in the first quarter of 2020. See Note 8 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for additional information on other Alleghany senior notes outstanding. (b) Alleghany Capital Operating Subsidiaries The debt associated with Alleghany Capital’s operating subsidiaries totaled $429.8 million and $367.1 million as of March 31, 2020 and December 31, 2019, respectively, and is generally used to support working capital needs and to help finance acquisitions. As of March 31, 2020, the $429.8 million included: • $161.6 million of borrowings by Jazwares under its available credit facilities and borrowings incurred and assumed from its acquisition of Wicked Cool Toys, LLC • $105.8 million of borrowings by W&W|AFCO Steel under its available credit facility and term loan, including borrowings incurred and assumed from its acquisition of Hirschfeld Holdings, LP in February 2018 ; • $74.2 • $51.2 $ • $37.0 None of these liabilities are guaranteed by Alleghany or Alleghany Capital. In December 2019, third-party, floating-rate term loans at Concord were repaid and replaced with approximately $33 million of intercompany floating-rate debt funded by the Alleghany parent company. The intercompany debt and related interest expenses are eliminated at the Alleghany consolidated level.

Summary of Significant Accoun_2

Summary of Significant Accounting Principles (Policies)	3 Months Ended
	Mar. 31, 2020
Accounting Policies [Abstract]
Principles of Financial Statement Presentation	(a) Principles of Financial Statement Presentation This Quarterly Report on Form 10-Q (this “Form 10-Q”) should be read in conjunction with the Annual Report on Form 10-K for the year ended December 31, 2019 (the “2019 Form 10-K”) of Alleghany Corporation (“Alleghany”). Alleghany Corporation, a Delaware corporation, owns and supports certain operating subsidiaries and manages investments, anchored by a core position in property and casualty reinsurance and insurance. Through its wholly-owned subsidiary Transatlantic Holdings, Inc. (“TransRe”), an Alleghany subsidiary since March 2012, Alleghany is engaged in the property and casualty reinsurance business. Through its wholly-owned subsidiary Alleghany Insurance Holdings LLC (“AIHL”), Alleghany is engaged in the property and casualty insurance business. AIHL’s insurance operations are principally conducted by its subsidiaries RSUI Group, Inc. (“RSUI”) and CapSpecialty, Inc. (“CapSpecialty”). RSUI and CapSpecialty have been subsidiaries of AIHL since July 2003 and January 2002, respectively. AIHL Re LLC (“AIHL Re”), a captive reinsurance company, which provides reinsurance to Alleghany’s current and former insurance operating subsidiaries and affiliates, has been a subsidiary of Alleghany since its formation in May 2006. Although Alleghany’s primary sources of revenues and earnings are its reinsurance and insurance operations and investments, Alleghany also generates revenues and expenses from a diverse portfolio of middle market businesses that are owned and managed through its wholly-owned subsidiary Alleghany Capital Corporation (“Alleghany Capital”). Alleghany Capital’s businesses include: • Precision Cutting Technologies, Inc. (“PCT”), a holding company headquartered in Rockford, Illinois, with four operating businesses: (i) Bourn & Koch, Inc., a provider of precision automated machine tool solutions; (ii) Diamond Technology Innovations, Inc., a manufacturer of waterjet orifices and nozzles and a provider of related services; (iii) Coastal Industrial Distributors, LLC, a provider of high-performance solid carbide end mills; and (iv) as of March 2020, Supermill LLC, a manufacturer of high-performance carbide end mills; • 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; • IPS-Integrated Project Services, LLC (“IPS”), a design, engineering, procurement, construction management and validation service provider focused on the global pharmaceutical and biotechnology industries, headquartered in Blue Bell, Pennsylvania; • Jazwares, LLC (together with its affiliates, “Jazwares”), a global toy, entertainment and musical instrument company, headquartered in Sunrise, Florida; • WWSC Holdings, LLC (“W&W|AFCO Steel”), a structural steel fabricator and erector, headquartered in Oklahoma City, Oklahoma; • CHECO Holdings, LLC (“Concord”), a hotel management and development company, headquartered in Raleigh, North Carolina; and • as of March 31, 2020, In addition, Alleghany owns certain other holding-company investments. Alleghany’s wholly-owned subsidiary Stranded Oil Resources Corporation (“SORC”) is an exploration and production company focused on enhanced oil recovery, headquartered in Golden, Colorado. Alleghany’s wholly-owned subsidiary Alleghany Properties Holdings LLC (“Alleghany Properties”) owns and manages certain properties in the Sacramento, California region. Alleghany’s public equity investments are managed primarily through Alleghany’s wholly-owned subsidiary Roundwood Asset Management LLC. Unless the context otherwise requires, references to “Alleghany” include Alleghany together with its subsidiaries. The accompanying consolidated financial statements include the results of Alleghany and its wholly-owned and majority-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). All material inter-company balances and transactions have been eliminated in consolidation. The portion of stockholders’ equity, net earnings and comprehensive income that is not attributable to Alleghany stockholders is presented on the consolidated balance sheets, the consolidated statements of earnings and comprehensive income and the consolidated statements of changes in stockholders’ equity as noncontrolling interests. Because all noncontrolling interests have the option to sell their ownership interests to Alleghany in the future (generally through 2024), the portion of stockholders’ equity that is not attributable to Alleghany stockholders is presented on the consolidated balance sheets and the consolidated statements of changes in stockholders’ equity as redeemable noncontrolling interests for all periods presented. In addition, Alleghany accretes the redeemable noncontrolling interests up to their future estimated redemption value over the period from the date of issuance to the earliest redemption date. The redemption value of the equity interests is generally based on the subsidiary’s earnings in specified periods preceding the applicable redemption date, calculated based on either a specified formula or an independent fair market valuation. During the first three months of 2020, the noncontrolling interests outstanding were approximately as follows: Kentucky Trailer - 23 percent; IPS - 15 percent; Jazwares - 25 percent; W&W|AFCO Steel - 20 percent; and Concord - 15 percent. Prior to April 1, 2019, the noncontrolling interests of PCT were approximately 11 percent. All noncontrolling interest holders of PCT have exercised their repurchase options and sold their ownership interests to PCT effective April 1, 2019. 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 as of 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 and reported revenues and expenses 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 statements of earnings and comprehensive income in the period in which the changes are made. (b) Other Significant Accounting Principles Alleghany’s significant accounting principles can be found in Note 1 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K.
Recent Accounting Standards	(c) Recent Accounting Standards Recently Adopted - 2019 In February 2016, the Financial Accounting Standards Board (the “FASB”) issued guidance on leases. Under this guidance, a lessee is required to recognize lease liabilities and corresponding right-of-use assets for leases with terms of more than one year, whereas under the prior guidance, a lessee was only required to recognize assets and liabilities for those leases qualifying as capital leases. This guidance also requires new disclosures about the amount, timing and uncertainty of cash flows arising from leases. The accounting by lessors remains largely unchanged. This guidance became effective in the first quarter of 2019 for public companies, with early adoption permitted. A modified retrospective transition approach was elected for all leases in existence as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements. Alleghany adopted this guidance in the first quarter of 2019 and the implementation did not have a material impact on its results of operations and financial condition. As part of its implementation, Alleghany elected to not separate lease components from non-lease components (such as office cleanings, security and maintenance services provided by Alleghany’s lessors for certain of its leases). Alleghany also elected the package of practical expedients under the transition guidance, which allowed Alleghany to not reevaluate existing lease classifications, among others. As of January 1, 2019, Alleghany’s adoption of this guidance resulted in recognition of an additional right-of-use asset of approximately $0.2 billion and a corresponding lease liability of $0.2 billion. See Note 12(b) to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for further information on Alleghany’s leases. In August 2017, the FASB issued guidance that simplifies the requirements to achieve hedge accounting, better reflects the economic results of hedging in financial statements and improves the alignment between hedge accounting and a company’s risk management activities. This guidance was effective in the first quarter of 2019 for public companies, with early adoption permitted. Alleghany adopted this guidance in the first quarter of 2019 and the implementation did not have a material impact on its results of operations and financial condition. Recently Adopted – 2020 In June 2016, the FASB issued guidance on credit losses. Under this guidance, a company is required to measure all expected credit losses on loans, reinsurance recoverables and other financial assets accounted for at cost or amortized cost, as applicable, over the remaining expected life of such assets. Estimates of expected credit losses are to be based on historical experience, current conditions and reasonable and supportable forecasts. Under former FASB guidance, credit losses on these assets generally required a company to recognize credit losses when it was probable that a loss had been incurred. Credit losses for securities accounted for on an available for sale (“AFS”) basis are to be measured in a manner similar to GAAP as applied under former guidance and cannot exceed the amount by which the fair value is less than the amortized cost, although the new guidance removes the length of time a security has been in an unrealized loss position as a possible indication of a credit impairment. Credit losses for all financial assets are to be recorded through an allowance for credit losses. Subsequent reversals in credit loss estimates are permitted and are to be recognized in earnings. This guidance also requires new disclosures about the significant estimates and judgments used in estimating credit losses, as well as the credit quality of financial assets. This guidance was effective in the first quarter of 2020 for public companies , with early adoption permitted. Alleghany adopt ed this guidance in the first quarter of 2020. As of January 1, 2020, Alleghany increased its allowances for credit losses on certain financial assets accounted for at cost or amortized cost by $4.5 million and recorded an after-tax cumulative effect reduction in opening retained earnings of $3.6 million. The increase in allowances for credit losses primarily relates to reinsurance recoverables and commercial mortgage loans. See Note 4 of this Form 10-Q for further information on Alleghany’s reinsurance recoverables and Note 3(i) of this Form 10-Q for further information on Alleghany’s investments in commercial mortgage loans. The increase in allowances for credit losses also relates to premium balances receivable. Estimates of expected credit losses for all of these assets are based on certain market-based indicators of credit quality, which are updated on an annual basis and re-assessed at least quarterly. Related credit loss expenses are recorded as a component of other operating expenses. As of January 1, 2020, credit losses for Alleghany’s AFS securities are recorded through an allowance for credit losses on the consolidated balance sheets and as credit losses for AFS securities in the consolidated statements of earnings and comprehensive income. See Note 3(f) of this Form 10-Q for further information on the credit quality for Alleghany’s AFS securities. In January 2017, the FASB issued guidance that simplifies the subsequent measurement of goodwill. Under this guidance, if an initial qualitative assessment indicates that the fair value of an operating subsidiary may be less than its carrying amount, an impairment charge is recognized for the amount by which the carrying amount of the operating subsidiary exceeds its estimated fair value. Any resulting impairment loss recognized cannot exceed the total amount of goodwill associated with the operating subsidiary. This guidance was effective in the first quarter of 2020 for public companies, with early adoption permitted. Alleghany adopted this guidance in the first quarter of 2020 and the implementation did not have a material impact on its results of operations and financial condition. See Note 2 to Notes to Consolidated Financial Statements set forth in Part II, Item 8, “Financial Statements and Supplementary Data” of the 2019 Form 10-K for further information on Alleghany’s goodwill. In August 2018, the FASB issued guidance that changes the financial statement disclosure requirements for measuring fair value. With respect to financial instruments classified as “Level 3” in the fair value disclosure hierarchy, the guidance requires certain additional disclosures for public companies related to amounts included in other comprehensive income and significant unobservable inputs used in the valuation, while removing disclosure requirements related to an entity’s overall valuation processes. The guidance also removes certain disclosure requirements related to transfers between financial instruments classified as “Level 1” and “Level 2” and provides clarification on certain other existing disclosure requirements. This guidance was effective for interim and annual periods beginning after December 15, 2019, with early adoption permitted with respect to any eliminated or modified disclosures. Alleghany adopted this guidance in the first quarter of 2020 and the implementation did not impact its results of operations or financial condition. See Note 2 of this Form 10-Q for further information on the fair value of Alleghany’s financial instruments. In March 2020, the FASB issued guidance to expedite and simplify the accounting associated with the anticipated migration away from the widely-used London Inter-bank Offered Rate and other similar rates as benchmark interest rates after 2021. Under pre-existing GAAP, such modifications made to: (i) loans and certain other contracts would require re-assessments of the accounting for those contacts, such as whether they were extinguished and remeasured from an accounting perspective; and (ii) derivative contracts may cause a change in accounting, such as a possible dedesignation of hedge accounting. This new guidance largely eliminates these requirements as a result of this migration to one or more new benchmark rates and is generally applicable for contract modifications made prior to December 31, 2022. Alleghany adopted this guidance in March 2020 and the implementation did not have an impact on its results of operations and financial condition.

Fair Value of Financial Instr_2

Fair Value of Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2020
Fair Value Disclosures [Abstract]
Carrying Value and Estimated Fair Value of Consolidated Financial Instruments	The following table presents the carrying value and estimated fair value of Alleghany’s consolidated financial instruments as of March 31, 2020 and December 31, 2019: March 31, 2020 December 31, 2019 Carrying Value Fair Value Carrying Value Fair Value ($ in millions) Assets Investments (excluding equity method investments and loans) (1) $ 15,584.6 $ 15,584.6 $ 17,632.3 $ 17,632.3 Liabilities Senior notes and other debt (2) $ 1,514.4 $ 1,674.8 $ 1,751.1 $ 1,962.7 (1) This table includes debt and equity securities, as well as partnership and non-marketable equity investments accounted for at fair value that are included in other invested assets. This table excludes investments accounted for using the equity method and commercial mortgage loans that are accounted for at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is disclosed below. (2) See Note 11 of this Form 10-Q for additional information on the senior notes and other debt.
Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs	The following tables present Alleghany’s financial instruments measured at fair value and the level of the fair value hierarchy of inputs used as of March 31, 2020 and December 31, 2019: Level 1 Level 2 Level 3 Total ($ in millions) As of March 31, 2020 Equity securities: Common stock $ 1,702.6 $ 3.5 $ — $ 1,706.1 Preferred stock — 2.1 2.0 4.1 Total equity securities 1,702.6 5.6 2.0 1,710.2 Debt securities: U.S. Government obligations — 846.3 — 846.3 Municipal bonds — 2,272.6 — 2,272.6 Foreign government obligations — 696.0 — 696.0 U.S. corporate bonds — 2,805.2 589.5 3,394.7 Foreign corporate bonds — 1,125.9 164.7 1,290.6 Mortgage and asset-backed securities: Residential mortgage-backed securities (“RMBS”) (1) — 1,697.4 2.5 1,699.9 Commercial mortgage-backed securities (“CMBS”) — 838.1 4.5 842.6 Other asset-backed securities (2) — 1,548.4 746.7 2,295.1 Total debt securities (3) — 11,829.9 1,507.9 13,337.8 Short-term investments — 536.1 — 536.1 Other invested assets (4) — — 0.5 0.5 Total investments (excluding equity method investments and loans) $ 1,702.6 $ 12,371.6 $ 1,510.4 $ 15,584.6 Senior notes and other debt $ — $ 1,245.0 $ 429.8 $ 1,674.8 Level 1 Level 2 Level 3 Total ($ in millions) As of December 31, 2019 Equity securities: Common stock $ 2,498.1 $ 3.3 $ — $ 2,501.4 Preferred stock — 2.1 2.0 4.1 Total equity securities 2,498.1 5.4 2.0 2,505.5 Debt securities: U.S. Government obligations — 1,215.0 — 1,215.0 Municipal bonds — 2,307.9 — 2,307.9 Foreign government obligations — 690.7 — 690.7 U.S. corporate bonds — 2,754.0 605.0 3,359.0 Foreign corporate bonds — 1,208.7 168.7 1,377.4 Mortgage and asset-backed securities: RMBS (1) — 1,838.5 1.9 1,840.4 CMBS — 865.9 5.8 871.7 Other asset-backed securities (2) — 1,692.9 856.7 2,549.6 Total debt securities — 12,573.6 1,638.1 14,211.7 Short-term investments — 914.8 — 914.8 Other invested assets (4) — — 0.3 0.3 Total investments (excluding equity method investments and loans) $ 2,498.1 $ 13,493.8 $ 1,640.4 $ 17,632.3 Senior notes and other debt $ — $ 1,595.6 $ 367.1 $ 1,962.7 (1) Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS. (2) Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively. (3) Includes allowance for credit losses as of March 31, 2020. See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities. ( 4 ) Includes partnership and non-marketable equity investments accounted for at fair value, and excludes investments accounted for using the equity method.
Reconciliations of Changes in Level Three Assets Measured at Fair Value	The following tables present reconciliations of the changes in Level 3 assets during the three months ended March 31, 2020 and 2019 measured at fair value: Debt Securities Mortgage and asset-backed Three Months Ended March 31, 2020 Preferred Stock U.S. Corporate Bonds Foreign Corporate Bonds RMBS CMBS Other Asset- backed Securities Other Invested Assets (1) Total ($ in millions) Balance as of January 1, 2020 $ 2.0 $ 605.0 $ 168.7 $ 1.9 $ 5.8 $ 856.7 $ 0.3 $ 1,640.4 Net realized/unrealized gains (losses) included in: Net earnings (losses) (2) — 0.2 — — — (0.1 ) — 0.1 Other comprehensive income (loss) — (18.8 ) (7.3 ) (0.2 ) (1.3 ) (101.4 ) 0.2 (128.8 ) Purchases — 6.7 7.7 0.9 — 43.6 — 58.9 Sales — — — — — (8.3 ) — (8.3 ) Issuances — — — — — — — — Settlements — (3.6 ) (1.1 ) (0.1 ) — (32.4 ) — (37.2 ) Transfers into Level 3 — — — — — — — — Transfers out of Level 3 — — (3.3 ) — — (11.4 ) — (14.7 ) Balance as of March 31, 2020 $ 2.0 $ 589.5 $ 164.7 $ 2.5 $ 4.5 $ 746.7 $ 0.5 $ 1,510.4 Debt Securities Mortgage and asset-backed Three Months Ended March 31, 2019 Preferred Stock U.S. Corporate Bonds Foreign Corporate Bonds CMBS Other Asset- backed Securities Other Invested Assets (1) Total ($ in millions) Balance as of January 1, 2019 $ 5.4 $ 425.7 $ 126.9 $ — $ 1,266.9 $ 0.7 $ 1,825.6 Net realized/unrealized gains (losses) included in: Net (losses) (2) — (9.9 ) — — — — (9.9 ) Other comprehensive income (loss) (0.8 ) 15.7 2.6 — 15.2 (0.2 ) 32.5 Purchases — 5.8 13.8 0.2 49.6 — 69.4 Sales — — (5.6 ) — (116.9 ) — (122.5 ) Issuances — — — — — — — Settlements — (2.6 ) (1.0 ) — (35.8 ) — (39.4 ) Transfers into Level 3 — — 14.7 — — — 14.7 Transfers out of Level 3 — (1.0 ) — — — — (1.0 ) Balance as of March 31, 2019 $ 4.6 $ 433.7 $ 151.4 $ 0.2 $ 1,179.0 $ 0.5 $ 1,769.4 (1) Includes partnership and non-marketable equity investments accounted for at fair value. (2) There were no credit losses recorded in net earnings related to Level 3 assets still held as of March 31, 2020 and 2019.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 2020
Investments [Abstract]
Amortized Cost and Fair Value of Available For Sale Securities	The following tables present the amortized cost and the fair value of AFS securities as of March 31, 2020 and December 31, 2019: Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Allowance for Credit Losses (2) Fair Value ($ in millions) As of March 31, 2020 Debt securities: U.S. Government obligations $ 793.5 $ 53.0 $ (0.2 ) $ — $ 846.3 Municipal bonds 2,158.3 117.2 (2.9 ) — 2,272.6 Foreign government obligations 669.1 27.6 (0.7 ) — 696.0 U.S. corporate bonds 3,386.8 103.5 (69.1 ) (26.5 ) 3,394.7 Foreign corporate bonds 1,311.2 14.5 (32.7 ) (2.4 ) 1,290.6 Mortgage and asset-backed securities: RMBS 1,613.7 86.9 (0.2 ) (0.5 ) 1,699.9 CMBS 870.6 15.9 (43.9 ) — 842.6 Other asset-backed securities (1) 2,517.6 5.6 (226.1 ) (2.0 ) 2,295.1 Total debt securities 13,320.8 424.2 (375.8 ) (31.4 ) 13,337.8 Short-term investments 536.1 — — — 536.1 Total investments $ 13,856.9 $ 424.2 $ (375.8 ) $ (31.4 ) $ 13,873.9 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value ($ in millions) As of December 31, 2019 Debt securities: U.S. Government obligations $ 1,198.5 $ 19.3 $ (2.8 ) $ 1,215.0 Municipal bonds 2,190.5 118.7 (1.3 ) 2,307.9 Foreign government obligations 675.9 16.4 (1.6 ) 690.7 U.S. corporate bonds 3,206.2 155.1 (2.3 ) 3,359.0 Foreign corporate bonds 1,348.1 32.2 (2.9 ) 1,377.4 Mortgage and asset-backed securities: RMBS 1,785.1 56.1 (0.8 ) 1,840.4 CMBS 849.9 23.2 (1.4 ) 871.7 Other asset-backed securities (1) 2,544.5 24.8 (19.7 ) 2,549.6 Total debt securities 13,798.7 445.8 (32.8 ) 14,211.7 Short-term investments 914.8 — — 914.8 Total investments $ 14,713.5 $ 445.8 $ (32.8 ) $ 15,126.5 (1) Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively. (2) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.
Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity	The following table presents the amortized cost and estimated fair value of debt securities by contractual maturity as of March 31, 2020. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties. Amortized Cost Fair Value ($ in millions) As of March 31, 2020 Short-term investments due in one year or less $ 536.1 $ 536.1 Mortgage and asset-backed securities (1) 5,001.9 4,837.6 Debt securities with maturity dates: One year or less 432.8 432.3 Over one through five years 3,174.0 3,201.3 Over five through ten years 2,728.0 2,770.8 Over ten years 1,984.1 2,095.8 Total debt securities $ 13,320.8 $ 13,337.8 (1) Mortgage and asset-backed securities by their nature do not generally have single maturity dates.
Net Investment Income	The following table presents net investment income for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Interest income $ 119.3 $ 119.2 Dividend income 10.1 9.8 Investment expenses (7.1 ) (7.5 ) Partnerships and other investment results (10.3 ) 1.5 Total $ 112.0 $ 123.0
Summary of Changes in Fair Value of Equity Securities	The following table presents changes in the fair value of equity securities for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Change in the fair value of equity securities sold during the period $ (164.9 ) $ 62.1 Change in the fair value of equity securities held at the end of the period (358.1 ) 330.3 Change in the fair value of equity securities $ (523.0 ) $ 392.4
Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses of Available For Sale Securities	The following table presents amounts of gross realized capital gains and gross realized capital losses for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Gross realized capital gains $ 59.0 $ 12.1 Gross realized capital losses (46.0 ) (7.7 ) Net realized capital gains $ 13.0 $ 4.4
Allowance for Credit Losses on Available for Sale Securities	The following table presents a rollforward of Alleghany’s allowance for credit losses for AFS securities for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) — Provision for credit losses 31.4 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 31.4 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.
Summary of Rating of Debt securities	The following table presents the ratings of Alleghany’s debt securities as of March 31, 2020 : Ratings as of March 31, 2020 AAA / Aaa AA / Aa A BBB / Baa Below BBB / Baa or Not Rated (1) Total ($ in millions) U.S. Government obligations $ — $ 846.3 $ — $ — $ — $ 846.3 Municipal bonds 184.2 1,526.5 489.4 71.6 0.9 2,272.6 Foreign government obligations 445.4 153.8 95.8 1.0 — 696.0 U.S. corporate bonds 9.0 158.0 1,586.4 1,298.2 343.1 3,394.7 Foreign corporate bonds 225.2 95.9 607.4 325.9 36.2 1,290.6 Mortgage and asset-backed securities: RMBS 2.5 1,692.3 — 1.0 4.1 1,699.9 CMBS 283.6 400.8 157.8 0.4 — 842.6 Other asset-backed securities 961.8 513.6 457.9 274.6 87.2 2,295.1 Total debt securities $ 2,111.7 $ 5,387.2 $ 3,394.7 $ 1,972.7 $ 471.5 $ 13,337.8 Percentage of debt securities, before allowance for credit losses 15.8 % 40.4 % 25.5 % 14.8 % 3.5 % 100.0 % (1) Consists of $109.3 million of securities rated BB / Ba, $187.5 million of securities rated B, $26.2 million of securities rated CCC, $0.4 million of securities rated CC, $2.1 million of securities rated below CC and $146.0 million of not rated securities.
Gross Unrealized Losses and Related Fair Values for AFS Securities for which an Allowance for Credit Losses has Not Been Recorded Grouped by Duration of Time in Continuous Unrealized Loss Position	The following tables present gross unrealized losses and related fair values for Alleghany’s AFS securities for which an allowance for credit losses has not been recorded, grouped by duration of time in a continuous unrealized loss position, as of March 31, 2020 and December 31, 2019: Less Than 12 Months 12 Months or More Total Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses ($ in millions) As of March 31, 2020 Debt securities: U.S. Government obligations $ 8.7 $ 0.2 $ — $ — $ 8.7 $ 0.2 Municipal bonds 129.4 2.9 0.1 — 129.5 2.9 Foreign government obligations 76.5 0.7 — — 76.5 0.7 U.S. corporate bonds 1,023.5 68.5 3.6 0.6 1,027.1 69.1 Foreign corporate bonds 656.5 32.6 17.4 0.1 673.9 32.7 Mortgage and asset-backed securities: RMBS 15.2 0.2 0.4 — 15.6 0.2 CMBS 436.4 43.9 0.2 — 436.6 43.9 Other asset-backed securities 1,529.0 139.5 491.3 86.6 2,020.3 226.1 Total temporarily impaired securities $ 3,875.2 $ 288.5 $ 513.0 $ 87.3 $ 4,388.2 $ 375.8 Less Than 12 Months 12 Months or More Total Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses ($ in millions) As of December 31, 2019 Debt securities: U.S. Government obligations $ 120.3 $ 2.0 $ 99.7 $ 0.8 $ 220.0 $ 2.8 Municipal bonds 90.2 1.3 4.3 — 94.5 1.3 Foreign government obligations 198.5 1.2 33.8 0.4 232.3 1.6 U.S. corporate bonds 113.8 1.6 27.6 0.7 141.4 2.3 Foreign corporate bonds 212.0 2.3 66.9 0.6 278.9 2.9 Mortgage and asset-backed securities: RMBS 48.7 0.3 48.6 0.5 97.3 0.8 CMBS 126.8 1.4 1.2 — 128.0 1.4 Other asset-backed securities 422.9 2.7 715.9 17.0 1,138.8 19.7 Total temporarily impaired securities $ 1,333.2 $ 12.8 $ 998.0 $ 20.0 $ 2,331.2 $ 32.8
Allowance for Credit Losses on Commercial Mortgage Loans	The following table presents a rollforward of Alleghany’s allowance for credit losses on commercial mortgage loans for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) 0.8 Provision for credit losses (2) 1.2 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 2.0 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. (2) Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

Reinsurance Ceded (Tables)

Reinsurance Ceded (Tables)	3 Months Ended
	Mar. 31, 2020
Insurance [Abstract]
Reinsurance Recoverables, Recorded After Allowance for Credit Losses	Amounts recoverable from reinsurers are recognized in a manner consistent with the loss and loss adjustment expense (“LAE”) liabilities associated with the reinsurance placement and presented on the balance sheet as reinsurance recoverables, and are recorded after an allowance for credit losses. Reinsurance recoverable balances as of March 31, 2020 and December 31, 2019 are presented in the table below: As of March 31, As of December 31, 2020 2019 ($ in millions) Reinsurance recoverables on paid losses $ 75.9 $ 98.1 Ceded outstanding loss and LAE 1,559.5 1,583.9 Reinsurance recoverables, before allowance for credit losses 1,635.4 1,682.0 Allowance for credit losses (6.8 ) — Total $ 1,628.6 $ 1,682.0
Information Regarding Concentration of Reinsurance Recoverables and Ratings Profile of Our Reinsurers	The following table presents information regarding concentration of Alleghany’s reinsurance recoverables and the ratings profile of our reinsurers as of March 31, 2020: Reinsurer (1) Rating (2) Amount Percentage ($ in millions) Syndicates at Lloyd's of London A (Excellent) $ 132.9 8.1 % Kane SAC Ltd, Rondout Segregated Account (3) not rated 121.8 7.4 % PartnerRe Ltd A (Excellent) 97.4 6.0 % Fairfax Financial Holdings Ltd A (Superior) 90.4 5.5 % Swiss Reinsurance Company A+ (Superior) 88.1 5.4 % Third Point Reinsurance Group A- (Excellent) 87.0 5.3 % Kane SAC Ltd, Bowery Segregated Account (3) not rated 77.4 4.7 % RenaissanceRe Holdings Ltd A+ (Superior) 75.4 4.6 % W.R. Berkley Corporation A+ (Superior) 67.7 4.1 % Liberty Mutual A (Excellent) 55.9 3.4 % All other reinsurers 741.4 45.5 % Total reinsurance recoverables, before allowance for credit losses (4) $ 1,635.4 100.0 % Allowance for credit losses (6.8 ) Total $ 1,628.6 Secured reinsurance recoverables (3) $ 659.6 40.3 % (1) Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company. (2) Represents the A.M. Best Company, Inc. financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due. (3) Represents reinsurance recoverables secured by funds held, trust agreements or letters of credit. (4) Approximately 66 percent of our reinsurance recoverables balance as of March 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit.
Allowance for Credit Losses on Reinsurance Recoverables	The following table presents a rollforward of Alleghany’s allowance for credit losses on reinsurance recoverables for the three months ended March 31, 2020: For the Three Months Ended March 31, 2020 Allowance for Credit Losses ($ in millions) Balance as of January 1, 2020 $ — Balance as of January 1, 2020 - cumulative effect of an accounting change (1) 3.3 Provision for credit losses (2) 3.5 Charge-offs — Recoveries — Balance as of March 31, 2020 $ 6.8 (1) See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance. (2) Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

Liability for Loss and LAE (Tab

Liability for Loss and LAE (Tables)	3 Months Ended
	Mar. 31, 2020
Insurance [Abstract]
Activity in Liability for Loss and Loss Adjustment Expense	The following table presents the activity in the liability for loss and LAE for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Reserves as of January 1 $ 11,928.4 $ 12,250.3 Less: reinsurance recoverables (1) 1,583.9 1,857.4 Net reserves as of January 1 10,344.5 10,392.9 Other adjustments — (0.2 ) Incurred loss and LAE, net of reinsurance, related to: Current year 1,086.4 829.9 Prior years (59.0 ) (37.4 ) Total incurred loss and LAE, net of reinsurance 1,027.4 792.5 Paid loss and LAE, net of reinsurance, related to: (2) Current year 87.3 75.3 Prior years 770.2 914.8 Total paid loss and LAE, net of reinsurance 857.5 990.1 Foreign currency exchange rate effect (7.5 ) 17.2 Net reserves as of March 31 10,506.9 10,212.3 Reinsurance recoverables as of March 31 (1) 1,559.5 1,642.8 Reserves as of March 31 $ 12,066.4 $ 11,855.1 (1) Reinsurance recoverables in this table include only ceded loss and LAE reserves. (2) Includes paid losses, net of reinsurance, related to commutations.
(Favorable) Unfavorable Prior Accident Year Loss Reserve Development	The following table presents the (favorable) unfavorable prior accident year loss reserve development for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Reinsurance Segment Property: Catastrophe events $ (32.3 ) (1) $ 8.4 (2) Non-catastrophe (14.4 ) (3) (19.1 ) (4) Total (46.7 ) (10.7 ) Casualty & other: Malpractice Treaties (5) — — Other (11.0 ) (6) (25.7 ) (7) Total (11.0 ) (25.7 ) Total Reinsurance Segment (57.7 ) (36.4 ) Insurance Segment RSUI: Casualty (1.0 ) (8) (5.6 ) (9) Property and other (0.5 ) (10) 1.9 (11) Total (1.5 ) (3.7 ) CapSpecialty 0.2 (12) 2.7 (12) Total incurred related to prior years $ (59.0 ) $ (37.4 ) (1) Primarily reflects favorable prior accident year loss reserve development related to Typhoon Hagibis in the 2019 accident year and catastrophes in the 2018 accident year. (2) Primarily reflects unfavorable prior accident year loss reserve development related to Typhoon Jebi in the 2018 accident year, partially offset by favorable prior accident year loss reserve development related to the wildfires in California in the 2018 accident year. (3) Primarily reflects favorable prior accident year loss reserve development in the 2017 and 2019 accident years. (4) Primarily reflects favorable prior accident year loss reserve development in the 2016 through 2018 accident years. (5) Represents certain malpractice treaties pursuant to which the increased underwriting profits created by the favorable prior accident year loss reserve development are largely retained by the cedants. As a result, the favorable prior accident year loss reserve development is largely offset by an increase in profit commission expense incurred when such favorable prior accident year loss reserve development occurs. (6) Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and earlier accident years, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2014 through 2019 accident years. (7) Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business in the 2012 through 2017 prior accident years and in the professional liability lines of business in the 2007 and prior accident years. ( 8 ) Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years. ( 9 ) Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in the 2017 accident year. (1 0 ) Primarily reflects favorable prior accident year loss reserve development related to catastrophes in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to catastrophes in the 2017 accident year. (1 1 ) Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Harvey in the 2017 accident year and Florence in the 2018 accident year. (1 2 ) Primarily reflects unfavorable prior accident year loss reserve development in certain specialty lines of business written through a program administrator in connection with a terminated program in the 2009 and 2010 accident years.

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	3 Months Ended
	Mar. 31, 2020
Stockholders Equity Note [Abstract]
Schedule of Common Stock Repurchases	The following table presents the shares of Common Stock that Alleghany repurchased in the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 Shares repurchased 62,540 129,567 Cost of shares repurchased (in millions) $ 44.3 $ 80.5 Average price per share repurchased $ 707.84 $ 621.19
Reconciliation of Accumulated Other Comprehensive Income (Loss)	The following tables present a reconciliation of the changes during the three months ended March 31, 2020 and 2019 in accumulated other comprehensive income (loss) attributable to Alleghany stockholders: Unrealized Appreciation of Investments Unrealized Currency Translation Adjustment Retirement Plans Total ($ in millions) Balance as of January 1, 2020 $ 321.0 $ (121.8 ) $ (27.9 ) $ 171.3 Other comprehensive income (loss), net of tax: Other comprehensive income (loss) before reclassifications (276.6 ) (13.7 ) 0.5 (289.8 ) Reclassifications from accumulated other comprehensive income (10.9 ) — — (10.9 ) Total (287.5 ) (13.7 ) 0.5 (300.7 ) Balance as of March 31, 2020 $ 33.5 $ (135.5 ) $ (27.4 ) $ (129.4 ) Unrealized Appreciation of Investments Unrealized Currency Translation Adjustment Retirement Plans Total ($ in millions) Balance as of January 1, 2019 $ (61.6 ) $ (124.7 ) $ (15.7 ) $ (202.0 ) Other comprehensive income (loss), net of tax: Other comprehensive income (loss) before reclassifications 183.4 (0.9 ) 0.9 183.4 Reclassifications from accumulated other comprehensive income 4.4 — — 4.4 Total 187.8 (0.9 ) 0.9 187.8 Balance as of March 31, 2019 $ 126.2 $ (125.6 ) $ (14.8 ) $ (14.2 )
Reclassifications of Accumulated Other Comprehensive Income (Loss)	The following table presents reclassifications out of accumulated other comprehensive income (loss) attributable to Alleghany stockholders during the three months ended March 31, 2020 and 2019: Accumulated Other Three Months Ended March 31, Comprehensive Income Component Line in Consolidated Statement of Earnings 2020 2019 ($ in millions) Unrealized appreciation of investments: Net realized capital gains (1) $ (45.1 ) $ (4.4 ) Credit losses for available for sale securities 31.4 10.0 Income taxes 2.8 (1.2 ) Total reclassifications: Net (earnings) losses $ (10.9 ) $ 4.4 (1) For the three months ended March 31, 2020, excludes a $7.1 million realized loss as a result of an early redemption of debt (see Note 11(a) of this Form 10-Q for further information), a $30.0 million impairment charge from a write-down of certain SORC assets

Earnings Per Share of Common _2

Earnings Per Share of Common Stock (Tables)	3 Months Ended
	Mar. 31, 2020
Earnings Per Share [Abstract]
Reconciliation of Earnings and Share Data used in Basic and Diluted (Losses) Earnings per Share Computations	The following table presents a reconciliation of the earnings and share data used in the basic and diluted earnings per share computations for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions, except share amounts) Net earnings available to Alleghany stockholders $ (361.2 ) $ 440.2 Effect of dilutive securities (3.0 ) — Income available to common stockholders for diluted earnings per share $ (364.2 ) $ 440.2 Weighted average common shares outstanding applicable to basic earnings per share 14,340,669 14,481,255 Effect of dilutive securities 15,439 3,981 Adjusted weighted average common shares outstanding applicable to diluted earnings per share 14,356,108 14,485,236 Contingently issuable shares (1) 18,703 39,325 (1) Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.

Segments of Business (Tables)

Segments of Business (Tables)	3 Months Ended
	Mar. 31, 2020
Segment Reporting [Abstract]
Results for Reportable Segments and Corporate Activities	The following tables present segment results for Alleghany’s three reportable segments and for corporate activities for the three months ended March 31, 2020 and 2019: Reinsurance Segment Insurance Segment Three Months Ended March 31, 2020 Property Casualty & other (1) Total RSUI Cap Specialty Total Subtotal Alleghany Capital Total Segments Corporate Activities Consolidated ($ in millions) Gross premiums written $ 412.6 $ 914.9 $ 1,327.5 $ 365.9 $ 85.5 $ 451.4 $ 1,778.9 $ — $ 1,778.9 $ (8.3 ) $ 1,770.6 Net premiums written 312.9 898.1 1,211.0 244.5 78.2 322.7 1,533.7 — 1,533.7 — 1,533.7 Net premiums earned 321.4 811.7 1,133.1 229.4 81.9 311.3 1,444.4 — 1,444.4 — 1,444.4 Net loss and LAE 247.9 614.3 862.2 116.0 49.2 165.2 1,027.4 — 1,027.4 — 1,027.4 Commissions, brokerage and other underwriting expenses 105.4 244.4 349.8 57.8 33.8 91.6 441.4 — 441.4 — 441.4 Underwriting (loss) profit (2) $ (31.9 ) $ (47.0 ) $ (78.9 ) $ 55.6 $ (1.1 ) $ 54.5 (24.4 ) — (24.4 ) — (24.4 ) Net investment income 109.7 1.8 111.5 0.5 112.0 Change in the fair value of equity securities (521.6 ) — (521.6 ) (1.4 ) (523.0 ) Net realized capital gains 45.1 4.9 50.0 (37.0 ) 13.0 Credit losses for available for sale securities (30.7 ) — (30.7 ) (0.7 ) (31.4 ) Noninsurance revenue 7.4 457.3 464.7 3.2 467.9 Other operating expenses 9.9 450.7 460.6 6.1 466.7 Corporate administration (2.0 ) — (2.0 ) (12.3 ) (14.3 ) Amortization of intangible assets 0.4 9.2 9.6 — 9.6 Interest expense 6.7 3.8 10.5 7.8 18.3 Earnings (losses) before income taxes $ (429.5 ) $ 0.3 $ (429.2 ) $ (37.0 ) $ (466.2 ) Reinsurance Segment Insurance Segment Three Months Ended March 31, 2019 Property Casualty & other (1) Total RSUI Cap Specialty Total Subtotal Alleghany Capital Total Segments Corporate Activities Consolidated ($ in millions) Gross premiums written $ 415.1 $ 816.5 $ 1,231.6 $ 299.0 $ 82.8 $ 381.8 $ 1,613.4 $ — $ 1,613.4 $ (6.8 ) $ 1,606.6 Net premiums written 316.2 791.4 1,107.6 201.4 76.7 278.1 1,385.7 — 1,385.7 — 1,385.7 Net premiums earned 308.9 720.3 1,029.2 192.4 75.7 268.1 1,297.3 — 1,297.3 — 1,297.3 Net loss and LAE 162.6 490.6 653.2 94.3 45.0 139.3 792.5 — 792.5 — 792.5 Commissions, brokerage and other underwriting expenses 103.7 232.3 336.0 53.5 30.6 84.1 420.1 — 420.1 — 420.1 Underwriting (loss) profit (2) $ 42.6 $ (2.6 ) $ 40.0 $ 44.6 $ 0.1 $ 44.7 84.7 — 84.7 — 84.7 Net investment income 117.9 1.3 119.2 3.8 123.0 Change in the fair value of equity securities 389.1 — 389.1 3.3 392.4 Net realized capital gains 4.1 0.3 4.4 — 4.4 Credit losses for available for sale securities (10.0 ) — (10.0 ) — (10.0 ) Noninsurance revenue 5.4 505.0 510.4 3.7 514.1 Other operating expenses 29.7 465.4 495.1 7.3 502.4 Corporate administration 1.1 — 1.1 18.3 19.4 Amortization of intangible assets 0.2 6.7 6.9 — 6.9 Interest expense 6.7 4.3 11.0 12.5 23.5 Earnings (losses) before income taxes $ 553.5 $ 30.2 $ 583.7 $ (27.3 ) $ 556.4 (1) Primarily consists of the following reinsurance lines of business: directors’ and officers’ liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; mortgage reinsurance; surety; and credit. (2) Underwriting profit represents net premiums earned less net loss and LAE and commissions, brokerage and other underwriting expenses, all as determined in accordance with GAAP, and does not include net investment income, change in the fair value of equity securities, net realized capital gains, credit losses for available for sale securities, noninsurance revenue, other operating expenses, corporate administration, amortization of intangible assets or interest expense. Underwriting profit does not replace earnings before income taxes determined in accordance with GAAP as a measure of profitability. Rather, Alleghany believes that underwriting profit enhances the understanding of its reinsurance and insurance segments’ operating results by highlighting net earnings attributable to their underwriting performance. Earnings before income taxes (a GAAP measure) may show a profit despite an underlying underwriting loss. Where underwriting losses persist over extended periods, a reinsurance or an insurance company’s ability to continue as an ongoing concern may be at risk. Therefore, Alleghany views underwriting profit as an important measure in the overall evaluation of performance.
Summary of Identifiable Assets and Equity	The following table presents identifiable assets, the portion of identifiable assets related to cash and invested assets and equity attributable to Alleghany for Alleghany’s reportable segments and for corporate activities as of March 31, 2020: Identifiable Assets Invested Assets and Cash Equity Attributable to Alleghany ($ in millions) Reinsurance segment $ 16,819.8 $ 13,077.2 $ 4,774.9 Insurance segment 6,414.1 4,697.6 2,377.5 Subtotal 23,233.9 17,774.8 7,152.4 Alleghany Capital 2,131.7 271.5 1,000.7 Total segments 25,365.6 18,046.3 8,153.1 Corporate activities 387.4 317.2 (296.0 ) Consolidated $ 25,753.0 $ 18,363.5 $ 7,857.1
Summary of Alleghany Capital Noninsurance Revenue	For Alleghany Capital’s industrial and non-industrial operations, noninsurance revenue consists of the sale of manufactured goods and services. The following table presents noninsurance revenue for the Alleghany Capital segment for the three months ended March 31, 2020 and 2019: Three Months Ended March 31, 2020 2019 ($ in millions) Industrial (1) $ 242.5 $ 260.1 Non-Industrial (2) 214.7 244.9 Corporate & other 0.1 — Alleghany Capital $ 457.3 $ 505.0 (1) For the three months ended March 31, 2020 and 2019, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time. (2) For the three months ended March 31, 2020 and 2019, approximately 82 percent 66 percent, respectively, of noninsurance revenue was recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.

Summary of Significant Accoun_3

Summary of Significant Accounting Principles - Additional Information (Detail) - USD ($) $ in Millions	Jan. 01, 2020	Apr. 01, 2020	Mar. 31, 2020	Mar. 31, 2019	Jan. 01, 2019
Accounting Standards Update 2016-02
Significant Accounting Policies [Line Items]
Right-of-use asset					$ 200
Lease liability					$ 200
Accounting Standards Update 2016-13
Significant Accounting Policies [Line Items]
Allowances for credit losses on certain financial assets	$ 4.5
After-tax cumulative effect reduction in opening retained earnings	$ (3.6)
Wilbert Funeral Services, Inc
Significant Accounting Policies [Line Items]
Equity interest percentage			45.00%
Subsequent Event | Wilbert Funeral Services, Inc
Significant Accounting Policies [Line Items]
Equity interest percentage		100.00%
Equity interest percentage acquired		55.00%
Kentucky Trailer
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			23.00%
Integrated Project Services LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			15.00%
Precision Cutting Technologies, Inc
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners				11.00%
Jazwares, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			25.00%
WWSC Holdings, LLC
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			20.00%
Concord
Significant Accounting Policies [Line Items]
Ownership of interest held by noncontrolling partners			15.00%

Carrying Values and Estimated F

Carrying Values and Estimated Fair Values of Consolidated Financial Instruments (Detail) - USD ($) $ in Millions		Mar. 31, 2020	Dec. 31, 2019
Assets
Investments (excluding equity method investments and loans)	[1]	$ 15,584.6	$ 17,632.3
Liabilities
Senior notes and other debt	[2]	1,674.8	1,962.7
Carrying Value
Assets
Investments (excluding equity method investments and loans)	[1]	15,584.6	17,632.3
Liabilities
Senior notes and other debt	[2]	$ 1,514.4	$ 1,751.1
[1]	This table includes debt and equity securities, as well as partnership and non-marketable equity investments accounted for at fair value that are included in other invested assets. This table excludes investments accounted for using the equity method and commercial mortgage loans that are accounted for at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is disclosed below.
[2]	See Note 11 of this Form 10-Q for additional information on the senior notes and other debt.

Financial Instruments Measured

Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		$ 1,710,193		$ 2,505,496
Estimated fair value of available for sale debt securities		13,337,754	[1]	14,211,745
Estimated fair value of investments (excluding equity method investments and loans)	[2]	15,584,600		17,632,300
Senior notes and other debt	[3]	1,674,800		1,962,700
Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		1,706,100		2,501,400
Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		4,100		4,100
U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		846,300		1,215,000
Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		2,272,600		2,307,900
Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		696,000		690,700
U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		3,394,700		3,359,000
Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		1,290,600		1,377,400
RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[4]	1,699,900		1,840,400
CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		842,600		871,700
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[5]	2,295,100		2,549,600
Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)		536,100		914,800
Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)	[6]	500		300
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		1,702,600		2,498,100
Estimated fair value of investments (excluding equity method investments and loans)		1,702,600		2,498,100
Level 1 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		1,702,600		2,498,100
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		5,600		5,400
Estimated fair value of available for sale debt securities		11,829,900	[1]	12,573,600
Estimated fair value of investments (excluding equity method investments and loans)		12,371,600		13,493,800
Senior notes and other debt		1,245,000		1,595,600
Level 2 | Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		3,500		3,300
Level 2 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,100		2,100
Level 2 | U.S. Government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		846,300		1,215,000
Level 2 | Municipal bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		2,272,600		2,307,900
Level 2 | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		696,000		690,700
Level 2 | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		2,805,200		2,754,000
Level 2 | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		1,125,900		1,208,700
Level 2 | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[4]	1,697,400		1,838,500
Level 2 | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		838,100		865,900
Level 2 | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[5]	1,548,400		1,692,900
Level 2 | Short-term Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)		536,100		914,800
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,000		2,000
Estimated fair value of available for sale debt securities		1,507,900	[1]	1,638,100
Estimated fair value of investments (excluding equity method investments and loans)		1,510,400		1,640,400
Senior notes and other debt		429,800		367,100
Level 3 | Preferred Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of equities		2,000		2,000
Level 3 | U.S. corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		589,500		605,000
Level 3 | Foreign corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		164,700		168,700
Level 3 | RMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[4]	2,500		1,900
Level 3 | CMBS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		4,500		5,800
Level 3 | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[5]	746,700		856,700
Level 3 | Other invested assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of investments (excluding equity method investments and loans)	[6]	$ 500		$ 300
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.
[2]	This table includes debt and equity securities, as well as partnership and non-marketable equity investments accounted for at fair value that are included in other invested assets. This table excludes investments accounted for using the equity method and commercial mortgage loans that are accounted for at unpaid principal balance. The fair value of short-term investments approximates amortized cost. The fair value of all other categories of investments is disclosed below.
[3]	See Note 11 of this Form 10-Q for additional information on the senior notes and other debt.
[4]	Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.
[5]	Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.
[6]	Includes partnership and non-marketable equity investments accounted for at fair value, and excludes investments accounted for using the equity method.

Financial Instruments Measure_2

Financial Instruments Measured at Fair Value and Level of Fair Value Hierarchy of Inputs (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 13,337,754	[1]	$ 14,211,745
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities	[2]	2,295,100		2,549,600
Collateralized loan obligations | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of available for sale debt securities		$ 732,300		$ 834,200
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.
[2]	Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.

Fair Value of Financial Instr_3

Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	$ 14.7	$ 1
Gross transfers into Level 3		14.7
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	3.3
Gross transfers into Level 3		14.7
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3		$ 1
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Gross transfers out of Level 3	$ 11.4

Reconciliations of Changes in L

Reconciliations of Changes in Level Three Assets Measured at Fair Value (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		$ 1,640.4	$ 1,825.6
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	0.1	(9.9)
Other comprehensive income (loss)		(128.8)	32.5
Purchases		58.9	69.4
Sales		(8.3)	(122.5)
Settlements		(37.2)	(39.4)
Transfers into Level 3			14.7
Transfers out of Level 3		(14.7)	(1)
Ending balance		1,510.4	1,769.4
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		2	5.4
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)			(0.8)
Ending balance		2	4.6
U.S. corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		605	425.7
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	0.2	(9.9)
Other comprehensive income (loss)		(18.8)	15.7
Purchases		6.7	5.8
Settlements		(3.6)	(2.6)
Transfers out of Level 3			(1)
Ending balance		589.5	433.7
Foreign corporate bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		168.7	126.9
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)		(7.3)	2.6
Purchases		7.7	13.8
Sales			(5.6)
Settlements		(1.1)	(1)
Transfers into Level 3			14.7
Transfers out of Level 3		(3.3)
Ending balance		164.7	151.4
RMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1.9
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)		(0.2)
Purchases		0.9
Settlements		(0.1)
Ending balance		2.5
CMBS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		5.8
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)		(1.3)
Purchases			0.2
Ending balance		4.5	0.2
Other invested assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	[2]	0.3	0.7
Net realized/unrealized gains (losses) included in:
Other comprehensive income (loss)	[2]	0.2	(0.2)
Ending balance	[2]	0.5	0.5
Other Asset Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		856.7	1,266.9
Net realized/unrealized gains (losses) included in:
Net earnings (losses)	[1]	(0.1)
Other comprehensive income (loss)		(101.4)	15.2
Purchases		43.6	49.6
Sales		(8.3)	(116.9)
Settlements		(32.4)	(35.8)
Transfers out of Level 3		(11.4)
Ending balance		$ 746.7	$ 1,179
[1]	There were no credit losses recorded in net earnings related to Level 3 assets still held as of March 31, 2020 and 2019.
[2]	Includes partnership and non-marketable equity investments accounted for at fair value.

Amortized Cost and Fair Value o

Amortized Cost and Fair Value of Available For Sale Securities (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		$ 13,320,808		$ 13,798,576
Debt securities, gross unrealized gains		424,200		445,800
Debt securities, gross unrealized losses		(375,800)		(32,800)
Debt securities, allowance for credit losses	[1]	(31,375)
Debt securities, fair value		13,337,754	[2]	14,211,745
Amortized Cost or Cost		13,856,900		14,713,500
Gross Unrealized Gains		424,200		445,800
Gross Unrealized Losses		(375,800)		(32,800)
Allowance for Credit Losses	[1]	(31,400)
Fair Value		13,873,900		15,126,500
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		793,500		1,198,500
Debt securities, gross unrealized gains		53,000		19,300
Debt securities, gross unrealized losses		(200)		(2,800)
Debt securities, fair value		846,300		1,215,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,158,300		2,190,500
Debt securities, gross unrealized gains		117,200		118,700
Debt securities, gross unrealized losses		(2,900)		(1,300)
Debt securities, fair value		2,272,600		2,307,900
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		669,100		675,900
Debt securities, gross unrealized gains		27,600		16,400
Debt securities, gross unrealized losses		(700)		(1,600)
Debt securities, fair value		696,000		690,700
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		3,386,800		3,206,200
Debt securities, gross unrealized gains		103,500		155,100
Debt securities, gross unrealized losses		(69,100)		(2,300)
Debt securities, allowance for credit losses	[1]	(26,500)
Debt securities, fair value		3,394,700		3,359,000
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,311,200		1,348,100
Debt securities, gross unrealized gains		14,500		32,200
Debt securities, gross unrealized losses		(32,700)		(2,900)
Debt securities, allowance for credit losses	[1]	(2,400)
Debt securities, fair value		1,290,600		1,377,400
RMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		1,613,700		1,785,100
Debt securities, gross unrealized gains		86,900		56,100
Debt securities, gross unrealized losses		(200)		(800)
Debt securities, allowance for credit losses	[1]	(500)
Debt securities, fair value	[3]	1,699,900		1,840,400
CMBS
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		870,600		849,900
Debt securities, gross unrealized gains		15,900		23,200
Debt securities, gross unrealized losses		(43,900)		(1,400)
Debt securities, fair value		842,600		871,700
Other Asset Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, amortized cost		2,517,600		2,544,500
Debt securities, gross unrealized gains		5,600		24,800
Debt securities, gross unrealized losses		(226,100)		(19,700)
Debt securities, allowance for credit losses	[1]	(2,000)
Debt securities, fair value	[4]	2,295,100		2,549,600
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost or Cost		536,100		914,800
Fair Value		$ 536,100		$ 914,800
[1]	See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.
[2]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.
[3]	Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.
[4]	Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.

Amortized Cost and Fair Value_2

Amortized Cost and Fair Value of Available For Sale Securities (Parenthetical) (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 13,337,754	[1]	$ 14,211,745
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities	[2]	2,295,100		2,549,600
Other asset-backed securities | Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value of available for sale debt securities		$ 732,300		$ 834,200
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.
[2]	Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.

Amortized Cost and Estimated Fa

Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Short-term investments due in one year or less, amortized cost		$ 536,100
Mortgage and asset-backed securities, amortized cost	[1]	5,001,900
Debt securities with maturity dates, amortized cost:
One year or less		432,800
Over one through five years		3,174,000
Over five through ten years		2,728,000
Over ten years		1,984,100
Debt securities, amortized cost		13,320,808		$ 13,798,576
Short-term investments due in one year or less, fair value		536,105		914,776
Mortgage and asset-backed securities, fair value	[1]	4,837,600
Debt securities with maturity dates, fair value:
One year or less		432,300
Over one through five years		3,201,300
Over five through ten years		2,770,800
Over ten years		2,095,800
Total debt securities, fair value		$ 13,337,754	[2]	$ 14,211,745
[1]	Mortgage and asset-backed securities by their nature do not generally have single maturity dates.
[2]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.

Net Investment Income (Detail)

Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Investment Income Net [Abstract]
Interest income	$ 119,300	$ 119,200
Dividend income	10,100	9,800
Investment expenses	(7,100)	(7,500)
Partnerships and other investment results	(10,300)	1,500
Net investment income	$ 111,982	$ 123,049

Summary of Changes in Fair Valu

Summary of Changes in Fair Value of Equity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Equity Securities Fv Ni Gain Loss [Abstract]
Change in the fair value of equity securities sold during the period	$ (164,900)	$ 62,100
Change in the fair value of equity securities held at the end of the period	(358,100)	330,300
Change in the fair value of equity securities	$ (522,976)	$ 392,353

Investments - Additional Inform

Investments - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended
	Dec. 31, 2012USD ($)	Mar. 31, 2020USD ($)Investment	Mar. 31, 2019USD ($)	Dec. 31, 2019USD ($)
Investments [Line Items]
Proceeds from sale of debt and equity securities		$ 2,600,000	$ 2,500,000
Realized loss on early redemption of debt		(7,100)
Realized gain on reduction of contingent consideration liabilities		$ 5,000
Securities impairment test description		Alleghany continually monitors the difference between amortized cost and the estimated fair value of its debt investments. The analysis of a security’s decline in value is performed in its functional currency.  Debt securities in an unrealized loss position are evaluated for credit losses if they meet any of the following criteria: (i) they are trading at a discount of at least 20 percent to amortized cost and have a credit rating below investment grade or are not rated; (ii) there has been a negative credit or news event with respect to the issuer that could indicate the existence of a credit loss; or (iii) Alleghany intends to sell, or it is more likely than not that Alleghany will sell, the debt security before recovery of its amortized cost basis. If Alleghany intends to sell, or it is more likely than not that Alleghany will sell, a debt security before recovery of its amortized cost basis, the total amount of the unrealized loss position is recognized as a credit loss in earnings. To the extent that a debt security that is in an unrealized loss position is not impaired based on the preceding, Alleghany will consider a debt security to be impaired when it believes it to be probable that Alleghany will not be able to collect the entire amortized cost basis. For debt securities in an unrealized loss position as of the end of each quarter, Alleghany develops a best estimate of the present value of expected cash flows. If the results of the cash flow analysis indicate that Alleghany will not recover the full amount of its amortized cost basis in the debt security, Alleghany records a credit loss in earnings equal to the difference between the present value of expected cash flows and the amortized cost basis of the debt security. If applicable, the difference between the total unrealized loss position on the debt security and the total loss recognized in earnings is the non-credit related portion, which is recorded as a component of other comprehensive income. In developing the cash flow analyses for debt securities, Alleghany considers various factors for the different categories of debt securities. For municipal bonds, Alleghany takes into account the taxing power of the issuer, source of revenue, credit risk and enhancements and pre-refunding. For mortgage and asset-backed securities, Alleghany discounts its best estimate of future cash flows at an effective rate equal to the original effective yield of the security or, in the case of floating rate securities, at the current coupon. Alleghany’s models include assumptions about prepayment speeds, default and delinquency rates, underlying collateral (if any), credit ratings, credit enhancements and other observable market data. For corporate bonds, Alleghany reviews business prospects, credit ratings and available information from asset managers and rating agencies for individual securities.
Credit losses for available for sale securities		$ 31,375
Credit losses for available for sale securities			$ 10,020
Other invested assets		541,512		$ 573,605
Commercial mortgage loans		682,655		$ 686,206
Commercial mortgage loan investments in default or in arrears		$ 0
Minimum
Investments [Line Items]
Term of commercial mortgage loans		2 years
Maximum
Investments [Line Items]
Term of commercial mortgage loans		10 years
Pillar Capital Holdings Limited And Managed Fund
Investments [Line Items]
Other invested assets		$ 194,700
Pillar Capital Holdings Limited And Managed Fund | Reinsurance Segment
Investments [Line Items]
Investment in other invested asset	$ 175,000
Pillar Capital Holdings Limited And Managed Fund | Insurance Segment
Investments [Line Items]
Investment in other invested asset	$ 25,000
Debt Securities
Investments [Line Items]
Number of securities in an unrealized loss position | Investment		1,672
Number of securities in an unrealized loss position for 12 months or more | Investment		126
Stranded Oil Resources Corporation
Investments [Line Items]
Write down of oil field assets		$ 30,000
Precision Cutting Technologies, Inc
Investments [Line Items]
Realized gain on reduction of contingent consideration liabilities		$ 5,000

Amounts of Gross Realized Capit

Amounts of Gross Realized Capital Gains and Gross Realized Capital Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Investments Debt And Equity Securities [Abstract]
Gross realized capital gains	$ 59,000	$ 12,100
Gross realized capital losses	(46,000)	(7,700)
Net realized capital gains	$ 13,026	$ 4,442

Allowance for Credit Losses on

Allowance for Credit Losses on Available for Sale Securities (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Credit losses for available for sale securities	$ 31,375
Ending balance	$ 31,375	[1]
[1]	See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.

Summary of Rating of Debt secur

Summary of Rating of Debt securities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020		Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 13,337,754	[1]	$ 14,211,745
Percentage of debt securities, before allowance for credit losses		100.00%
AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 2,111,700
Percentage of debt securities, before allowance for credit losses		15.80%
AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 5,387,200
Percentage of debt securities, before allowance for credit losses		40.40%
A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 3,394,700
Percentage of debt securities, before allowance for credit losses		25.50%
BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 1,972,700
Percentage of debt securities, before allowance for credit losses		14.80%
Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	$ 471,500
Percentage of debt securities, before allowance for credit losses	[2]	3.50%
U.S. Government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		$ 846,300		1,215,000
U.S. Government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		846,300
Municipal bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,272,600		2,307,900
Municipal bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		184,200
Municipal bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,526,500
Municipal bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		489,400
Municipal bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		71,600
Municipal bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	900
Foreign government obligations
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		696,000		690,700
Foreign government obligations | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		445,400
Foreign government obligations | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		153,800
Foreign government obligations | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		95,800
Foreign government obligations | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
U.S. corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		3,394,700		3,359,000
U.S. corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		9,000
U.S. corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		158,000
U.S. corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,586,400
U.S. corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,298,200
U.S. corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	343,100
Foreign corporate bonds
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,290,600		1,377,400
Foreign corporate bonds | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		225,200
Foreign corporate bonds | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		95,900
Foreign corporate bonds | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		607,400
Foreign corporate bonds | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		325,900
Foreign corporate bonds | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	36,200
RMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[3]	1,699,900		1,840,400
RMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		2,500
RMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,692,300
RMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		1,000
RMBS | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	4,100
CMBS
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		842,600		871,700
CMBS | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		283,600
CMBS | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		400,800
CMBS | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		157,800
CMBS | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		400
Other asset-backed securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[4]	2,295,100		$ 2,549,600
Other asset-backed securities | AAA / Aaa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		961,800
Other asset-backed securities | AA / Aa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		513,600
Other asset-backed securities | A
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		457,900
Other asset-backed securities | BBB / Baa
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities		274,600
Other asset-backed securities | Below BBB / Baa or Not Rated
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	[2]	$ 87,200
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.
[2]	Consists of $109.3 million of securities rated BB / Ba, $187.5 million of securities rated B, $26.2 million of securities rated CCC, $0.4 million of securities rated CC, $2.1 million of securities rated below CC and $146.0 million of not rated securities.
[3]	Primarily includes government agency pass-through securities guaranteed by a government agency or government sponsored enterprise, among other types of RMBS.
[4]	Includes $732.3 million and $834.2 million of collateralized loan obligations as of March 31, 2020 and December 31, 2019, respectively.

Summary of Rating of Debt sec_2

Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 13,337,754	[1]	$ 14,211,745
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	109,300
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	187,500
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	26,200
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	400
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	2,100
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 146,000
[1]	Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.

Gross Unrealized Losses and Rel

Gross Unrealized Losses and Related Fair Values for AFS Securities for which an Allowance for Credit Losses has Not Been Recorded Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,875.2	$ 1,333.2
Securities, less than 12 months, gross unrealized losses	288.5	12.8
Securities, 12 months or more, fair value	513	998
Securities, 12 months or more, gross unrealized losses	87.3	20
Total, fair value	4,388.2	2,331.2
Total, gross unrealized losses	375.8	32.8
U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	8.7	120.3
Securities, less than 12 months, gross unrealized losses	0.2	2
Securities, 12 months or more, fair value		99.7
Securities, 12 months or more, gross unrealized losses		0.8
Total, fair value	8.7	220
Total, gross unrealized losses	0.2	2.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	129.4	90.2
Securities, less than 12 months, gross unrealized losses	2.9	1.3
Securities, 12 months or more, fair value	0.1	4.3
Total, fair value	129.5	94.5
Total, gross unrealized losses	2.9	1.3
Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	76.5	198.5
Securities, less than 12 months, gross unrealized losses	0.7	1.2
Securities, 12 months or more, fair value		33.8
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value	76.5	232.3
Total, gross unrealized losses	0.7	1.6
U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,023.5	113.8
Securities, less than 12 months, gross unrealized losses	68.5	1.6
Securities, 12 months or more, fair value	3.6	27.6
Securities, 12 months or more, gross unrealized losses	0.6	0.7
Total, fair value	1,027.1	141.4
Total, gross unrealized losses	69.1	2.3
Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	656.5	212
Securities, less than 12 months, gross unrealized losses	32.6	2.3
Securities, 12 months or more, fair value	17.4	66.9
Securities, 12 months or more, gross unrealized losses	0.1	0.6
Total, fair value	673.9	278.9
Total, gross unrealized losses	32.7	2.9
RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	15.2	48.7
Securities, less than 12 months, gross unrealized losses	0.2	0.3
Securities, 12 months or more, fair value	0.4	48.6
Securities, 12 months or more, gross unrealized losses		0.5
Total, fair value	15.6	97.3
Total, gross unrealized losses	0.2	0.8
CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	436.4	126.8
Securities, less than 12 months, gross unrealized losses	43.9	1.4
Securities, 12 months or more, fair value	0.2	1.2
Total, fair value	436.6	128
Total, gross unrealized losses	43.9	1.4
Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,529	422.9
Securities, less than 12 months, gross unrealized losses	139.5	2.7
Securities, 12 months or more, fair value	491.3	715.9
Securities, 12 months or more, gross unrealized losses	86.6	17
Total, fair value	2,020.3	1,138.8
Total, gross unrealized losses	$ 226.1	$ 19.7

Allowance for Credit Losses o_2

Allowance for Credit Losses on Commercial Mortgage Loans (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Provision for credit losses	$ 1.2	[1]
Ending balance	2
Accounting Standards Update 2016-13
Financing Receivable Allowance For Credit Losses [Line Items]
Beginning balance	$ 0.8	[2]
[1]	Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.
[2]	See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.

Reinsurance Ceded - Additional

Reinsurance Ceded - Additional Information (Detail) - Reinsurance Segment	3 Months Ended
	Mar. 31, 2020
Effects Of Reinsurance [Line Items]
Retrocession protection term	1 year
Term of catastrophe bonds	4 years

Reinsurance Recoverables, Recor

Reinsurance Recoverables, Recorded After Allowance for Credit Losses (Detail) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Reinsurance recoverables on paid losses		$ 75,900	$ 98,100
Ceded outstanding loss and LAE	[1]	1,559,500	1,583,900	$ 1,642,800	$ 1,857,400
Reinsurance recoverables, before allowance for credit losses		1,635,400	1,682,000
Allowance for credit losses		(6,800)
Total		$ 1,628,649	$ 1,681,962
[1]	Reinsurance recoverables in this table include only ceded loss and LAE reserves.

Information Regarding Concentra

Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses		$ 1,635,400	$ 1,682,000
Allowance for credit losses		(6,800)
Reinsurance recoverables		1,628,649	$ 1,681,962
Reinsurance Recoverable | Reinsurer Concentration Risk
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[1],[2]	$ 1,635,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[1],[2]	100.00%
Allowance for credit losses	[2]	$ (6,800)
Reinsurance recoverables	[2]	$ 1,628,600
Reinsurance Recoverable | Reinsurer Concentration Risk | Ceded Credit Risk, Secured
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3]	40.30%
Reinsurance recoverables	[2],[3]	$ 659,600
Reinsurance Recoverable | Reinsurer Concentration Risk | All other reinsurers
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2]	$ 741,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2]	45.50%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Syndicates at Lloyd's of London
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 132,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	8.10%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | PartnerRe Limited
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 97,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	6.00%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Fairfax Financial Holdings Limited
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 90,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.50%
AM Best, A Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Liberty Mutual
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 55,900
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	3.40%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd, Rondout Segregated Account
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 121,800
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	7.40%
Not Rated | Reinsurance Recoverable | Reinsurer Concentration Risk | Kane SAC Ltd, Bowery Segregated Account
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[3],[4]	$ 77,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[3],[4]	4.70%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Swiss Reinsurance Company
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 88,100
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.40%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | RenaissanceRe Holdings Ltd
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 75,400
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.60%
AM Best, A+ Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | W.R. Berkley Corporation
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 67,700
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	4.10%
AM Best, A- Rating | Reinsurance Recoverable | Reinsurer Concentration Risk | Third Point Reinsurance Group
Ceded Credit Risk [Line Items]
Reinsurance recoverables, before allowance for credit losses	[2],[4]	$ 87,000
Reinsurance recoverable as percentage of total reinsurance recoverables	[2],[4]	5.30%
[1]	Approximately 66 percent of our reinsurance recoverables balance as of March 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit.
[2]	Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company.
[3]	Represents reinsurance recoverables secured by funds held, trust agreements or letters of credit.
[4]	Represents the A.M. Best Company, Inc. financial strength rating for the applicable reinsurance subsidiary or subsidiaries from which the reinsurance recoverable is due.

Information Regarding Concent_2

Information Regarding Concentration of Reinsurance Recoverables and The Ratings Profile of Our Reinsurers (Parenthetical) (Detail) - Reinsurance Recoverable - Reinsurer Concentration Risk	3 Months Ended
	Mar. 31, 2020
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	100.00%	[1],[2]
Reinsurers Rated A Or Higher
Ceded Credit Risk [Line Items]
Reinsurance recoverable as percentage of total reinsurance recoverables	66.00%
[1]	Approximately 66 percent of our reinsurance recoverables balance as of March 31, 2020 was due from reinsurers having an A.M. Best Company, Inc. financial strength rating of A (Excellent) or higher, with a majority of the other reinsurance recoverables being secured by funds held, trust agreements or letters of credit.
[2]	Reinsurance recoverables reflect amounts due from one or more reinsurance subsidiaries of the listed company.

Allowance for Credit Losses o_3

Allowance for Credit Losses on Reinsurance Recoverables (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Allowance for Credit Losses
Provision for credit losses	$ 3.5	[1]
Balance as of March 31, 2020	6.8
Accounting Standards Update 2016-13
Allowance for Credit Losses
Balance as of January 1, 2020	$ 3.3	[2]
[1]	Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.
[2]	See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.

Activity in the Liability for L

Activity in the Liability for Loss and Loss Adjustment Expense (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Activity in liability for loss and loss adjustment expense
Reserves as of January 1		$ 11,928,359	$ 12,250,300
Less: reinsurance recoverables	[1]	1,583,900	1,857,400
Net reserves as of January 1		10,344,500	10,392,900
Other adjustments			(200)
Incurred loss and LAE, net of reinsurance, related to:
Current year		1,086,400	829,900
Prior years		(59,000)	(37,400)
Total incurred loss and LAE, net of reinsurance		1,027,444	792,498
Paid loss and LAE, net of reinsurance, related to:
Current year	[2]	87,300	75,300
Prior years	[2]	770,200	914,800
Total paid loss and LAE, net of reinsurance	[2]	857,500	990,100
Foreign currency exchange rate effect		(7,500)	17,200
Net reserves as of March 31		10,506,900	10,212,300
Reinsurance recoverables as of March 31	[1]	1,559,500	1,642,800
Reserves as of March 31		$ 12,066,438	$ 11,855,100
[1]	Reinsurance recoverables in this table include only ceded loss and LAE reserves.
[2]	Includes paid losses, net of reinsurance, related to commutations.

(Favorable) Unfavorable Prior A

(Favorable) Unfavorable Prior Accident Year Loss Reserve Development (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020		Mar. 31, 2019
Claims Development [Line Items]
Claims incurred related to prior years		$ (59)		$ (37.4)
Reinsurance Segment
Claims Development [Line Items]
Claims incurred related to prior years		(57.7)		(36.4)
Reinsurance Segment | Property
Claims Development [Line Items]
Claims incurred related to prior years		(46.7)		(10.7)
Reinsurance Segment | Property | Property Catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(32.3)	[1]	8.4	[2]
Reinsurance Segment | Property | Property Non-catastrophe
Claims Development [Line Items]
Claims incurred related to prior years		(14.4)	[3]	(19.1)	[4]
Reinsurance Segment | Casualty & Other
Claims Development [Line Items]
Claims incurred related to prior years		(11)		(25.7)
Reinsurance Segment | Casualty & Other | All Other Casualty and Other
Claims Development [Line Items]
Claims incurred related to prior years		(11)	[5]	(25.7)	[6]
Insurance Segment | RSUI
Claims Development [Line Items]
Claims incurred related to prior years		(1.5)		(3.7)
Insurance Segment | RSUI | Casualty Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(1)	[7]	(5.6)	[8]
Insurance Segment | RSUI | Property and Other Insurance
Claims Development [Line Items]
Claims incurred related to prior years		(0.5)	[9]	1.9	[10]
Insurance Segment | CapSpecialty Incorporated
Claims Development [Line Items]
Claims incurred related to prior years	[11]	$ 0.2		$ 2.7
[1]	Primarily reflects favorable prior accident year loss reserve development related to Typhoon Hagibis in the 2019 accident year and catastrophes in the 2018 accident year.
[2]	Primarily reflects unfavorable prior accident year loss reserve development related to Typhoon Jebi in the 2018 accident year, partially offset by favorable prior accident year loss reserve development related to the wildfires in California in the 2018 accident year.
[3]	Primarily reflects favorable prior accident year loss reserve development in the 2017 and 2019 accident years.
[4]	Primarily reflects favorable prior accident year loss reserve development in the 2016 through 2018 accident years.
[5]	Primarily reflects favorable prior accident year loss reserve development in the longer-tailed lines of business in the 2013 and earlier accident years, partially offset by unfavorable prior accident year development in the shorter-tailed lines of business in the 2014 through 2019 accident years.
[6]	Primarily reflects favorable prior accident year loss reserve development in the shorter-tailed lines of business in the 2012 through 2017 prior accident years and in the professional liability lines of business in the 2007 and prior accident years.
[7]	Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in recent accident years.
[8]	Primarily reflects favorable prior accident year loss reserve development in the directors’ and officers’ liability and umbrella/excess lines of business in the 2011 through 2015 accident years, partially offset by unfavorable prior accident year loss reserve development in the professional liability lines of business in the 2017 accident year.
[9]	Primarily reflects favorable prior accident year loss reserve development related to catastrophes in the 2018 accident year, partially offset by unfavorable prior accident year loss reserve development related to catastrophes in the 2017 accident year.
[10]	Primarily reflects unfavorable prior accident year loss reserve development related to Hurricanes Harvey in the 2017 accident year and Florence in the 2018 accident year.
[11]	Primarily reflects unfavorable prior accident year loss reserve development in certain specialty lines of business written through a program administrator in connection with a terminated program in the 2009 and 2010 accident years.

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Tax Disclosure [Abstract]
Income tax expense (benefit) related to CARES Act 2020	$ 6.9
Effective income tax rate	22.40%	19.50%
Interest or penalties accrued for uncertain tax positions	$ 0

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail) - USD ($)	Mar. 15, 2020	Feb. 29, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2018
Stockholders Equity Note [Line Items]
Remaining authorized repurchases under share repurchase program			$ 582,900,000
Par value of common stock			$ 1
Dividends paid			$ 215,013,000
Dividend Declared in 2020
Stockholders Equity Note [Line Items]
Special dividend declared per share		$ 15
Dividend record date		Mar. 5, 2020
Dividend payable nature			special
Dividends paid	$ 215,000,000
Stock Repurchase Programs 2019
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase				$ 500,000,000
Stock Repurchase Programs 2018
Stockholders Equity Note [Line Items]
Aggregate amount of common stock authorized for repurchase					$ 400,000,000

Schedule of Common Stock Repurc

Schedule of Common Stock Repurchases (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Stockholders Equity Note [Abstract]
Shares repurchased	62,540	129,567
Cost of shares repurchased	$ 44,268	$ 80,486
Average price per share repurchased	$ 707.84	$ 621.19

Reconciliation of Accumulated O

Reconciliation of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 8,776,734	$ 7,692,710
Other comprehensive income (loss), net of tax:
Ending Balance	7,857,062	8,236,487
Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	321,000	(61,600)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	(276,600)	183,400
Reclassifications from accumulated other comprehensive income	(10,900)	4,400
Total	(287,500)	187,800
Ending Balance	33,500	126,200
Unrealized Currency Translation Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(121,800)	(124,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	(13,700)	(900)
Total	(13,700)	(900)
Ending Balance	(135,500)	(125,600)
Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(27,900)	(15,700)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	500	900
Total	500	900
Ending Balance	(27,400)	(14,800)
Accumulated Other Comprehensive Income (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	171,350	(202,003)
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss) before reclassifications	(289,800)	183,400
Reclassifications from accumulated other comprehensive income	(10,900)	4,400
Total	(300,700)	187,800
Ending Balance	$ (129,343)	$ (14,157)

Reclassifications of Accumulate

Reclassifications of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains		$ (13,026)	$ (4,442)
Credit losses for available for sale securities		31,375
Credit losses for available for sale securities			10,020
Income taxes		(104,720)	108,512
Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications		(10,900)	4,400
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net realized capital gains	[1]	(45,100)	(4,400)
Credit losses for available for sale securities		31,400
Credit losses for available for sale securities			10,000
Income taxes		$ 2,800	$ (1,200)
[1]	For the three months ended March 31, 2020, excludes a $7.1 million realized loss as a result of an early redemption of debt (see Note 11(a) of this Form 10-Q for further information), a $30.0 million impairment charge from a write-down of certain SORC assets

Reclassifications of Accumula_2

Reclassifications of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020USD ($)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early redemption of debt	$ (7.1)
Realized gain on reduction of contingent consideration liabilities	5
Alleghany Corporation | 2020 Senior Notes
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Loss on early redemption of debt	(7.1)
Stranded Oil Resources Corporation
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Write down of oil field assets	$ 30

Reconciliation of Earnings and

Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net earnings available to Alleghany stockholders		$ (361,218)	$ 440,227
Effect of dilutive securities		(3,000)
Income available to common stockholders for diluted earnings per share		$ (364,200)	$ 440,200
Weighted average common shares outstanding applicable to basic earnings per share		14,340,669	14,481,255
Effect of dilutive securities		15,439	3,981
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		14,356,108	14,485,236
Contingently issuable shares	[1]	18,703	39,325
[1]	Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.

Segments of Business - Addition

Segments of Business - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2020Segment
Segment Reporting Information [Line Items]
Number of reportable segments	3
Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Equity interest percentage	45.00%

Results for Reportable Segments

Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,770,600	$ 1,606,600
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,420	1,297,309
Net loss and LAE		1,027,444	792,498
Commissions, brokerage and other underwriting expenses		441,396	420,126
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		111,982	123,049
Change in the fair value of equity securities		(522,976)	392,353
Net realized capital gains		13,026	4,442
Credit losses for available for sale securities		(31,375)
Credit losses for available for sale securities			(10,020)
Noninsurance revenue		467,849	514,123
Other operating expenses		466,661	502,418
Corporate administration		(14,307)	19,369
Amortization of intangible assets		9,568	6,900
Interest expense		18,356	23,531
(Losses) earnings before income taxes		(466,192)	556,414
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,778,900	1,613,400
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,400	1,297,300
Net loss and LAE		1,027,400	792,500
Commissions, brokerage and other underwriting expenses		441,400	420,100
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		111,500	119,200
Change in the fair value of equity securities		(521,600)	389,100
Net realized capital gains		50,000	4,400
Credit losses for available for sale securities		(30,700)
Credit losses for available for sale securities			(10,000)
Noninsurance revenue		464,700	510,400
Other operating expenses		460,600	495,100
Corporate administration		(2,000)	1,100
Amortization of intangible assets		9,600	6,900
Interest expense		10,500	11,000
(Losses) earnings before income taxes		(429,200)	583,700
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,327,500	1,231,600
Net premiums written		1,211,000	1,107,600
Net premiums earned		1,133,100	1,029,200
Net loss and LAE		862,200	653,200
Commissions, brokerage and other underwriting expenses		349,800	336,000
Underwriting (loss) profit	[1]	(78,900)	40,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		412,600	415,100
Net premiums written		312,900	316,200
Net premiums earned		321,400	308,900
Net loss and LAE		247,900	162,600
Commissions, brokerage and other underwriting expenses		105,400	103,700
Underwriting (loss) profit	[1]	(31,900)	42,600
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	914,900	816,500
Net premiums written	[2]	898,100	791,400
Net premiums earned	[2]	811,700	720,300
Net loss and LAE	[2]	614,300	490,600
Commissions, brokerage and other underwriting expenses	[2]	244,400	232,300
Underwriting (loss) profit	[1],[2]	(47,000)	(2,600)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		451,400	381,800
Net premiums written		322,700	278,100
Net premiums earned		311,300	268,100
Net loss and LAE		165,200	139,300
Commissions, brokerage and other underwriting expenses		91,600	84,100
Underwriting (loss) profit	[1]	54,500	44,700
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		365,900	299,000
Net premiums written		244,500	201,400
Net premiums earned		229,400	192,400
Net loss and LAE		116,000	94,300
Commissions, brokerage and other underwriting expenses		57,800	53,500
Underwriting (loss) profit	[1]	55,600	44,600
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		85,500	82,800
Net premiums written		78,200	76,700
Net premiums earned		81,900	75,700
Net loss and LAE		49,200	45,000
Commissions, brokerage and other underwriting expenses		33,800	30,600
Underwriting (loss) profit	[1]	(1,100)	100
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,778,900	1,613,400
Net premiums written		1,533,700	1,385,700
Net premiums earned		1,444,400	1,297,300
Net loss and LAE		1,027,400	792,500
Commissions, brokerage and other underwriting expenses		441,400	420,100
Underwriting (loss) profit	[1]	(24,400)	84,700
Net investment income		109,700	117,900
Change in the fair value of equity securities		(521,600)	389,100
Net realized capital gains		45,100	4,100
Credit losses for available for sale securities		(30,700)
Credit losses for available for sale securities			(10,000)
Noninsurance revenue		7,400	5,400
Other operating expenses		9,900	29,700
Corporate administration		(2,000)	1,100
Amortization of intangible assets		400	200
Interest expense		6,700	6,700
(Losses) earnings before income taxes		(429,500)	553,500
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income		1,800	1,300
Net realized capital gains		4,900	300
Noninsurance revenue		457,300	505,000
Other operating expenses		450,700	465,400
Amortization of intangible assets		9,200	6,700
Interest expense		3,800	4,300
(Losses) earnings before income taxes		300	30,200
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(8,300)	(6,800)
Net investment income		500	3,800
Change in the fair value of equity securities		(1,400)	3,300
Net realized capital gains		(37,000)
Credit losses for available for sale securities		(700)
Noninsurance revenue		3,200	3,700
Other operating expenses		6,100	7,300
Corporate administration		(12,300)	18,300
Interest expense		7,800	12,500
(Losses) earnings before income taxes		$ (37,000)	$ (27,300)
[1]	Underwriting profit represents net premiums earned less net loss and LAE and commissions, brokerage and other underwriting expenses, all as determined in accordance with GAAP, and does not include net investment income, change in the fair value of equity securities, net realized capital gains, credit losses for available for sale securities, noninsurance revenue, other operating expenses, corporate administration, amortization of intangible assets or interest expense. Underwriting profit does not replace earnings before income taxes determined in accordance with GAAP as a measure of profitability. Rather, Alleghany believes that underwriting profit enhances the understanding of its reinsurance and insurance segments’ operating results by highlighting net earnings attributable to their underwriting performance. Earnings before income taxes (a GAAP measure) may show a profit despite an underlying underwriting loss. Where underwriting losses persist over extended periods, a reinsurance or an insurance company’s ability to continue as an ongoing concern may be at risk. Therefore, Alleghany views underwriting profit as an important measure in the overall evaluation of performance.
[2]	Primarily consists of the following reinsurance lines of business: directors’ and officers’ liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; mortgage reinsurance; surety; and credit.

Summary of Identifiable Assets

Summary of Identifiable Assets and Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Identifiable Assets	$ 25,752,941	$ 26,931,604
Invested Assets and Cash	18,363,500
Equity Attributable to Alleghany	7,857,062	$ 8,776,734	$ 8,236,487	$ 7,692,710
Operating Segments
Segment Reporting Information [Line Items]
Identifiable Assets	25,365,600
Invested Assets and Cash	18,046,300
Equity Attributable to Alleghany	8,153,100
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	16,819,800
Invested Assets and Cash	13,077,200
Equity Attributable to Alleghany	4,774,900
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	6,414,100
Invested Assets and Cash	4,697,600
Equity Attributable to Alleghany	2,377,500
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Identifiable Assets	23,233,900
Invested Assets and Cash	17,774,800
Equity Attributable to Alleghany	7,152,400
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Identifiable Assets	2,131,700
Invested Assets and Cash	271,500
Equity Attributable to Alleghany	1,000,700
Corporate activities
Segment Reporting Information [Line Items]
Identifiable Assets	387,400
Invested Assets and Cash	317,200
Equity Attributable to Alleghany	$ (296,000)

Summary of Alleghany Capital No

Summary of Alleghany Capital Non-Insurance Revenue (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Other Revenues [Line Items]
Noninsurance revenue		$ 467,849	$ 514,123
Operating Segments
Other Revenues [Line Items]
Noninsurance revenue		464,700	510,400
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Noninsurance revenue		457,300	505,000
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[1]	242,500	260,100
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Noninsurance revenue	[2]	214,700	$ 244,900
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Noninsurance revenue		$ 100
[1]	For the three months ended March 31, 2020 and 2019, the vast majority of noninsurance revenue was recognized as goods and services transferred to customers over time.
[2]	For the three months ended March 31, 2020 and 2019, approximately 82 percent 66 percent, respectively, of noninsurance revenue was recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.

Summary of Alleghany Capital _2

Summary of Alleghany Capital Non-Insurance Revenue (Parenthetical) (Detail)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating Segments | Alleghany Capital Corporation Segment | Non-Industrial Segment | Transferred over Time
Other Revenues [Line Items]
Noninsurance revenue percentage	82.00%	66.00%

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Thousands	Jan. 15, 2020	Sep. 20, 2010	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Debt Instrument [Line Items]
Interest paid			$ 17,065	$ 19,368
Loss on early redemption of debt			(7,100)
Senior Notes and other debt			1,514,381		$ 1,751,113
Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			$ 429,800		367,100
Alleghany Corporation | 2020 Senior Notes
Debt Instrument [Line Items]
Senior notes, face value		$ 300,000
Senior notes, maturity date		Sep. 15, 2020
Senior notes, frequency of interest payment			Interest on the 2020 Senior Notes was payable semi-annually on March 15 and September 15 of each year.
Senior notes, interest rate		5.625%
Senior notes, issuance rate		99.60%
Proceeds from issuance of Senior notes		$ 298,900
Senior notes, effective yield		5.67%
Cost of senior note redemption	$ 312,700
Aggregate principal amount redeemed	300,000
Redemption premium	7,100
Interest paid	$ 5,600
Loss on early redemption of debt			$ (7,100)
Jazwares, LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			161,600
WWSC Holdings, LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			105,800
Kentucky Trailer | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			74,200
Integrated Project Services LLC | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			51,200
Precision Cutting Technologies, Inc | Operating Segments | Alleghany Capital Corporation Segment
Debt Instrument [Line Items]
Senior Notes and other debt			$ 37,000
Concord | Operating Segments | Alleghany Capital Corporation Segment | Floating Rate Debt Funded by Alleghany Corporation
Debt Instrument [Line Items]
Senior Notes and other debt					$ 33,000