Reclassifications of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized capital gains																	$ 232,119		$ 157,879	$ 127,141
Other than temporary impairment losses																	(44,047)		(2,907)	(3,607)
Income taxes																	(225,882)		(17,032)	(47,586)
Net earnings	205,300	[1]	113,200	[1]	113,700	[1]	196,300	[1]	(92,600)	[1]	125,400	[1]	109,300	[1]	560,100	[1]	628,421		702,244	143,251
Reclassification out of Accumulated Other Comprehensive Income | Unrealized Appreciation of Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized capital gains																	190,300	[2]
Other than temporary impairment losses																	(44,000)
Income taxes																	(51,200)
Net earnings																	$ 95,100

[1] Attributable to Alleghany stockholders.

[2] Excludes the following items not reclassified out of accumulated other comprehensive income: (i) a $46.8 million realized capital gain related to the sale of Homesite, and (ii) a $5.0 million realized capital loss relating to a non-cash impairment charge related to certain finite-lived intangible assets at Capitol.