Segment Results for Reportable Segments and Corporate Activities (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Disclosure [Line Items]
Gross premiums written	$ 1,301,100,000		$ 1,237,500,000
Net premiums written	1,134,000,000		1,093,900,000
Net premiums earned	1,053,997,000		1,075,013,000
Net loss and LAE	611,159,000		567,413,000
Commissions, brokerage and other underwriting expenses	324,236,000		326,227,000
Underwriting profit (loss)	118,600,000	[1]	181,400,000	[1]
Net investment income	110,583,000		118,811,000
Net realized capital gains	96,836,000		50,902,000
Other than temporary impairment losses	(5,220,000)		(32,312,000)
Other income	30,446,000		11,141,000
Other operating expenses	53,342,000		30,738,000
Corporate administration	9,632,000		12,422,000
Amortization of intangible assets	(1,861,000)		11,630,000
Interest expense	21,811,000		21,736,000
Earnings (losses) before income taxes	268,323,000		253,389,000
Reinsurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	941,300,000		907,700,000
Net premiums written	879,800,000		866,000,000
Net premiums earned	793,300,000		853,900,000
Net loss and LAE	463,900,000		471,200,000
Commissions, brokerage and other underwriting expenses	239,400,000		251,300,000
Underwriting profit (loss)	90,000,000	[1]	131,400,000	[1]
Reinsurance Segment | Property
Segment Reporting Disclosure [Line Items]
Gross premiums written	298,600,000		270,100,000
Net premiums written	250,600,000		236,700,000
Net premiums earned	234,500,000		250,100,000
Net loss and LAE	61,300,000		56,200,000
Commissions, brokerage and other underwriting expenses	65,300,000		62,400,000
Underwriting profit (loss)	107,900,000	[1]	131,500,000	[1]
Reinsurance Segment | Casualty & Other
Segment Reporting Disclosure [Line Items]
Gross premiums written	642,700,000	[2]	637,600,000	[2]
Net premiums written	629,200,000	[2]	629,300,000	[2]
Net premiums earned	558,800,000	[2]	603,800,000	[2]
Net loss and LAE	402,600,000	[2]	415,000,000	[2]
Commissions, brokerage and other underwriting expenses	174,100,000	[2]	188,900,000	[2]
Underwriting profit (loss)	(17,900,000)	[1],[2]	(100,000)	[1],[2]
Insurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	367,200,000		335,100,000
Net premiums written	254,200,000		227,900,000
Net premiums earned	260,700,000		221,100,000
Net loss and LAE	147,300,000		96,200,000
Commissions, brokerage and other underwriting expenses	84,800,000		74,900,000
Underwriting profit (loss)	28,600,000	[1]	50,000,000	[1]
Insurance Segment | RSUI
Segment Reporting Disclosure [Line Items]
Gross premiums written	302,300,000		285,800,000
Net premiums written	195,400,000		181,200,000
Net premiums earned	204,000,000		175,700,000
Net loss and LAE	112,400,000		70,400,000
Commissions, brokerage and other underwriting expenses	54,800,000		48,400,000
Underwriting profit (loss)	36,800,000	[1]	56,900,000	[1]
Insurance Segment | Capitol
Segment Reporting Disclosure [Line Items]
Gross premiums written	49,100,000		40,200,000
Net premiums written	43,300,000		37,800,000
Net premiums earned	42,300,000		37,300,000
Net loss and LAE	24,000,000		18,100,000
Commissions, brokerage and other underwriting expenses	22,000,000		19,500,000
Underwriting profit (loss)	(3,700,000)	[1]	(300,000)	[1]
Insurance Segment | PCC
Segment Reporting Disclosure [Line Items]
Gross premiums written	15,800,000	[3]	9,100,000	[3]
Net premiums written	15,500,000	[3]	8,900,000	[3]
Net premiums earned	14,400,000	[3]	8,100,000	[3]
Net loss and LAE	10,900,000	[3]	7,700,000	[3]
Commissions, brokerage and other underwriting expenses	8,000,000	[3]	7,000,000	[3]
Underwriting profit (loss)	(4,500,000)	[1],[3]	(6,600,000)	[1],[3]
Corporate activities
Segment Reporting Disclosure [Line Items]
Gross premiums written	(7,400,000)	[4]	(5,300,000)	[4]
Net investment income	1,400,000	[4]	25,500,000	[4]
Net realized capital gains	21,700,000	[4]	1,600,000	[4]
Other income	27,000,000	[4]	10,500,000	[4]
Other operating expenses	32,000,000	[4]	10,400,000	[4]
Corporate administration	9,600,000	[4]	12,400,000	[4]
Amortization of intangible assets	100,000	[4]
Interest expense	9,500,000	[4]	9,400,000	[4]
Earnings (losses) before income taxes	(1,100,000)	[4]	5,400,000	[4]
Total Segments
Segment Reporting Disclosure [Line Items]
Gross premiums written	1,308,500,000		1,242,800,000
Net premiums written	1,134,000,000		1,093,900,000
Net premiums earned	1,054,000,000		1,075,000,000
Net loss and LAE	611,200,000		567,400,000
Commissions, brokerage and other underwriting expenses	324,200,000		326,200,000
Underwriting profit (loss)	118,600,000	[1]	181,400,000	[1]
Net investment income	109,200,000		93,300,000
Net realized capital gains	75,100,000		49,300,000
Other than temporary impairment losses	(5,200,000)		(32,300,000)
Other income	3,400,000		700,000
Other operating expenses	21,300,000		20,400,000
Amortization of intangible assets	(1,900,000)		11,600,000
Interest expense	12,300,000		12,400,000
Earnings (losses) before income taxes	$ 269,400,000		$ 248,000,000

[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, net realized capital gains, OTTI losses, other income, other operating expenses, 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 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 assumed 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; surety; and credit.

[3] Includes underwriting results of AIHL Re.

[4] Includes elimination of minor reinsurance activity between segments.