Reinsurance Ceded - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Ceded loss and loss adjustment expenses incurred	$ 325.8	$ 309.6	$ 250.3
AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company | Alleghany Corporation | Financial Guarantee
Effects of Reinsurance [Line Items]
Guarantee amount related to intercompany reinsurance agreement	100.0
AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company | Pacific Compensation Insurance Company
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 100.0
Accident year stop-loss coverage initiation point	75.00%
AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company | Minimum | Pacific Compensation Insurance Company
Effects of Reinsurance [Line Items]
Accident year covered by stop-loss coverage	2013
AIHL Re Limited Liability Company reinsurance of Pacific Compensation Insurance Company | Maximum | Pacific Compensation Insurance Company
Effects of Reinsurance [Line Items]
Accident year covered by stop-loss coverage	2017
AIHL Re Limited Liability Company reinsurance of CapSpecialty Incorporated | 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 December 31, 2016, the catastrophe reinsurance program consisted of three layers, with portions of the first and second layers placed on May 1, 2015 and May 1, 2016 and portions of the third layer placed on May 1, 2014 and May 1, 2016. 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 the following expiration terms: approximately 34 percent of coverage limits, which originally expired on April 30, 2016 and was renewed May 1, 2016, currently expires on April 30, 2019; approximately 33 percent of coverage limits expire on April 30, 2017; and approximately 33 percent of coverage limits expire on April 30, 2018. The third layer of coverage originally expired on April 30, 2017. However, effective May 1, 2016, approximately 39 percent of the third layer of coverage was cancelled and replaced with the same coverage for a three year period expiring on April 30, 2019. The remaining coverage limits 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, 2016. On May 1, 2016, the property per risk program was renewed and the new program will expire on April 30, 2017. For the 2016 to 2017 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 | 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 2
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2015
Reinsurance coverage expiration date	Apr. 30, 2017
Percentage of reinsurance program layer	33.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 1 | Period 3
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2015
Reinsurance coverage expiration date	Apr. 30, 2018
Percentage of reinsurance program layer	33.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 1 | Period 1
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2016
Reinsurance coverage expiration date	Apr. 30, 2019
Percentage of reinsurance program layer	34.00%
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 2
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2015
Reinsurance coverage expiration date	Apr. 30, 2017
Percentage of reinsurance program layer	33.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 2 | Period 3
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2015
Reinsurance coverage expiration date	Apr. 30, 2018
Percentage of reinsurance program layer	33.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 2 | Period 1
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2016
Reinsurance coverage expiration date	Apr. 30, 2019
Percentage of reinsurance program layer	34.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 3
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 200.0
Net retention	$ 600.0
RSUI | Current Catastrophe Reinsurance Program | Layer 3 | Period 2
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2016
Reinsurance coverage expiration date	Apr. 30, 2019
Percentage of reinsurance program layer	39.00%
RSUI | Current Catastrophe Reinsurance Program | Layer 3 | Period 1
Effects of Reinsurance [Line Items]
Reinsurance coverage initiation date	May 01, 2014
Reinsurance coverage expiration date	Apr. 30, 2017
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 initiation date	May 01, 2016
Reinsurance coverage expiration date	Apr. 30, 2017
CapSpecialty Incorporated | AIHL Re Limited Liability Company reinsurance of CapSpecialty Incorporated
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 50.0