Reinsurance Ceded - Additional Information (Detail) - RSUI $ in Millions	9 Months Ended
Sep. 30, 2016 USD ($)
Effects of Reinsurance [Line Items]
Description of reinsurance program	RSUI’s catastrophe reinsurance program covers catastrophe risks including, among others, windstorms and earthquakes. Portions of the catastrophe reinsurance program include multi-year terms, some of which were entered into in 2014. As of September 30, 2016, the catastrophe reinsurance program consisted of three layers, with portions of the first two 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 portions of the program expiring on April 30, 2016 were renewed on 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 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.
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
Current Catastrophe Reinsurance Program
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 600.0
Net retention	200.0
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%
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%
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%
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%
Current Catastrophe Reinsurance Program \| Layer 2
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 100.0
Net retention	$ 500.0
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%
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%
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%
Current Catastrophe Reinsurance Program \| Layer 3
Effects of Reinsurance [Line Items]
Reinsurance coverage before co-participation	$ 200.0
Net retention	$ 600.0
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%
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

Reinsurance Ceded - Additional Information (Detail) - RSUI
$ in Millions

9 Months Ended

Sep. 30, 2016

USD ($)

RSUI’s catastrophe reinsurance program covers catastrophe risks including, among others, windstorms and earthquakes. Portions of the catastrophe reinsurance program include multi-year terms, some of which were entered into in 2014. As of September 30, 2016, the catastrophe reinsurance program consisted of three layers, with portions of the first two 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 portions of the program expiring on April 30, 2016 were renewed on 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 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.