Reinsurance Ceded - Additional Information (Detail) - USD ($) $ in Millions			9 Months Ended
	May. 01, 2015	May. 01, 2014	Sep. 30, 2015
AIHL Re Limited Liability Company Intercompany Reinsurance Program | Alleghany Corporation | Financial Guarantee
Effects of Reinsurance [Line Items]
Guarantee amount related to intercompany reinsurance agreement			$ 50.0
RSUI
Effects of Reinsurance [Line Items]
Description of reinsurance program			RSUI’s catastrophe reinsurance program covers catastrophe risks including, among others, windstorms and earthquakes. As of May 1, 2015, the catastrophe reinsurance program consists of three layers with the first two layers placed on May 1, 2015 and the third layer placed on May 1, 2014. Portions of the catastrophe reinsurance program include multi-year terms, some of which were entered into in 2014. The catastrophe reinsurance program provides coverage for $600.0 million of losses in excess of a $200.0 million net retention after application of surplus share treaties and facultative reinsurance. The first layer provides coverage for $300.0 million of losses, subject to a 5.0 percent co-participation by RSUI, in excess of $200.0 million, the second layer provides coverage for $100.0 million of losses in excess of $500.0 million, with no co-participation by RSUI, and the third layer provides coverage for $200.0 million of losses in excess of $600.0 million, with no co-participation by RSUI. The first and second layers of coverage include expiration terms as follows: 34.0 percent of coverage limits will expire on April 30, 2016; 33.0 percent of coverage limits will expire on April 30, 2017; and 33.0 percent of coverage limits will expire on April 30, 2018. The third layer of coverage will expire on April 30, 2017. In addition, RSUI’s property per risk reinsurance program runs on an annual basis from May 1 to the following April 30 and thus expired on April 30, 2015. On May 1, 2015, the property per risk program was renewed and will expire on April 30, 2016. For the 2015 to 2016 period, RSUI’s property per risk reinsurance program provides coverage for $90.0 million of losses, subject to a 10.0 percent co-participation by RSUI, in excess of a $10.0 million net retention per risk after application of surplus share treaties and facultative reinsurance.
RSUI | Property Per Risk
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 90.0
Net retention	$ 10.0
Percentage of co-participation	10.00%
Reinsurance coverage expiration date	Apr. 30, 2016
RSUI | Current Catastrophe Reinsurance Program
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 600.0
Net retention	200.0
RSUI | Current Catastrophe Reinsurance Program | Layer 1
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	300.0
Net retention	$ 200.0
Percentage of co-participation	5.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 1 | Period 1
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	34.00%
Reinsurance coverage expiration date	Apr. 30, 2016
RSUI | Current Catastrophe Reinsurance Program | Layer 1 | Period 2
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	33.00%
Reinsurance coverage expiration date	Apr. 30, 2017
RSUI | Current Catastrophe Reinsurance Program | Layer 1 | Period 3
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	33.00%
Reinsurance coverage expiration date	Apr. 30, 2018
RSUI | Current Catastrophe Reinsurance Program | Layer 2
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 100.0
Net retention	$ 500.0
RSUI | Current Catastrophe Reinsurance Program | Layer 2 | Period 1
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	34.00%
Reinsurance coverage expiration date	Apr. 30, 2016
RSUI | Current Catastrophe Reinsurance Program | Layer 2 | Period 2
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	33.00%
Reinsurance coverage expiration date	Apr. 30, 2017
RSUI | Current Catastrophe Reinsurance Program | Layer 2 | Period 3
Effects of Reinsurance [Line Items]
Percentage of reinsurance program layer	33.00%
Reinsurance coverage expiration date	Apr. 30, 2018
RSUI | Current Catastrophe Reinsurance Program | Layer 3
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation		$ 200.0
Net retention		$ 600.0
Reinsurance coverage expiration date		Apr. 30, 2017
CapSpecialty Incorporated | AIHL Re Limited Liability Company Intercompany Reinsurance Program
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation			$ 50.0