Segment Results for Reportable Segments and Corporate Activities (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Disclosure [Line Items]
Gross premiums written	$ 1,258,900,000		$ 1,301,100,000
Net premiums written	1,091,100,000		1,134,000,000
Net premiums earned	1,017,171,000		1,053,997,000
Net loss and LAE	546,916,000		611,159,000
Commissions, brokerage and other underwriting expenses	343,609,000		324,236,000
Underwriting profit (loss)	126,700,000	[1]	118,600,000	[1]
Net investment income	113,382,000		110,583,000
Net realized capital gains	43,151,000		96,836,000
Other than temporary impairment losses	(52,281,000)		(5,220,000)
Other income	36,200,000		30,446,000
Other operating expenses	67,405,000		53,342,000
Corporate administration	12,678,000		9,632,000
Amortization of intangible assets	(1,660,000)		(1,861,000)
Interest expense	23,092,000		21,811,000
Earnings before income taxes	165,583,000		268,323,000
Operating Segments
Segment Reporting Disclosure [Line Items]
Gross premiums written	1,266,400,000		1,308,500,000
Net premiums written	1,091,100,000		1,134,000,000
Net premiums earned	1,017,200,000		1,054,000,000
Net loss and LAE	546,900,000		611,200,000
Commissions, brokerage and other underwriting expenses	343,600,000		324,200,000
Underwriting profit (loss)	126,700,000	[1]	118,600,000	[1]
Net investment income	110,800,000		109,200,000
Net realized capital gains	48,800,000		75,100,000
Other than temporary impairment losses	(52,300,000)		(5,200,000)
Other income	1,400,000		3,400,000
Other operating expenses	26,600,000		21,300,000
Corporate administration	300,000
Amortization of intangible assets	(1,800,000)		(1,900,000)
Interest expense	9,700,000		12,300,000
Earnings before income taxes	200,600,000		269,400,000
Operating Segments | Reinsurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	902,200,000		941,300,000
Net premiums written	828,800,000		879,800,000
Net premiums earned	746,900,000		793,300,000
Net loss and LAE	406,400,000		463,900,000
Commissions, brokerage and other underwriting expenses	256,700,000		239,400,000
Underwriting profit (loss)	83,800,000	[1]	90,000,000	[1]
Operating Segments | Reinsurance Segment | Property
Segment Reporting Disclosure [Line Items]
Gross premiums written	271,600,000		298,600,000
Net premiums written	210,100,000		250,600,000
Net premiums earned	215,600,000		234,500,000
Net loss and LAE	63,200,000		61,300,000
Commissions, brokerage and other underwriting expenses	66,900,000		65,300,000
Underwriting profit (loss)	85,500,000	[1]	107,900,000	[1]
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Disclosure [Line Items]
Gross premiums written	630,600,000	[2]	642,700,000	[2]
Net premiums written	618,700,000	[2]	629,200,000	[2]
Net premiums earned	531,300,000	[2]	558,800,000	[2]
Net loss and LAE	343,200,000	[2]	402,600,000	[2]
Commissions, brokerage and other underwriting expenses	189,800,000	[2]	174,100,000	[2]
Underwriting profit (loss)	(1,700,000)	[1],[2]	(17,900,000)	[1],[2]
Operating Segments | Insurance Segment
Segment Reporting Disclosure [Line Items]
Gross premiums written	364,200,000		367,200,000
Net premiums written	262,300,000		254,200,000
Net premiums earned	270,300,000		260,700,000
Net loss and LAE	140,500,000		147,300,000
Commissions, brokerage and other underwriting expenses	86,900,000		84,800,000
Underwriting profit (loss)	42,900,000	[1]	28,600,000	[1]
Operating Segments | Insurance Segment | RSUI
Segment Reporting Disclosure [Line Items]
Gross premiums written	288,200,000		302,300,000
Net premiums written	190,500,000		195,400,000
Net premiums earned	203,100,000		204,000,000
Net loss and LAE	100,800,000		112,400,000
Commissions, brokerage and other underwriting expenses	55,300,000		54,800,000
Underwriting profit (loss)	47,000,000	[1]	36,800,000	[1]
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Disclosure [Line Items]
Gross premiums written	54,500,000		49,100,000
Net premiums written	50,600,000		43,300,000
Net premiums earned	47,600,000		42,300,000
Net loss and LAE	24,400,000		24,000,000
Commissions, brokerage and other underwriting expenses	22,800,000		22,000,000
Underwriting profit (loss)	400,000	[1]	(3,700,000)	[1]
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Disclosure [Line Items]
Gross premiums written	21,500,000	[3]	15,800,000	[3]
Net premiums written	21,200,000	[3]	15,500,000	[3]
Net premiums earned	19,600,000	[3]	14,400,000	[3]
Net loss and LAE	15,300,000	[3]	10,900,000	[3]
Commissions, brokerage and other underwriting expenses	8,800,000	[3]	8,000,000	[3]
Underwriting profit (loss)	(4,500,000)	[1],[3]	(4,500,000)	[1],[3]
Corporate activities
Segment Reporting Disclosure [Line Items]
Gross premiums written	(7,500,000)	[4]	(7,400,000)	[4]
Net investment income	2,600,000	[4]	1,400,000	[4]
Net realized capital gains	(5,700,000)	[4]	21,700,000	[4]
Other income	34,800,000	[4]	27,000,000	[4]
Other operating expenses	40,800,000	[4]	32,000,000	[4]
Corporate administration	12,400,000	[4]	9,600,000	[4]
Amortization of intangible assets	100,000	[4]	100,000	[4]
Interest expense	13,400,000	[4]	9,500,000	[4]
Earnings before income taxes	$ (35,000,000)	[4]	$ (1,100,000)	[4]

[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, 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 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.