Organization and Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	9 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) Insurance	Sep. 30, 2017 USD ($)	Jun. 07, 2018	Jan. 01, 2018 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Number of insurance companies | Insurance		2
Corporate income tax rate		21.00%
Provisional transition tax	$ 2,600,000
Change in provisional transition tax		$ 1,300,000
Decrease net cash used in investing activities		$ (127,120,000)	$ (131,399,000)
Accounting Standards Update 2016-01
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment from adoption					$ 0
Accounting Standards Update 2016-01 | Retained Earnings
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment from adoption					7,700,000
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive Income (Loss)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment from adoption					(7,700,000)
Accounting Standards Update 2016-18
Organization And Summary Of Significant Accounting Policies [Line Items]
Decrease net cash used in investing activities			76,800,000
Reclassification of restricted and unrestricted cash and cash equivalent	71,300,000
Accounting Standards Update 2018-02 | Retained Earnings
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment from adoption					(2,800,000)
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income (Loss)
Organization And Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment from adoption					$ 2,800,000
Immaterial Reclassification Correction
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash reclassified from short- term investment to cash and cash equivalent	20,500,000
Oversee deposit reclassified from short-term investments to other invested asset	$ 28,800,000
Decrease net cash used in investing activities			$ 9,200,000
Bracht, Deckers & Mackelbert NV and Assurances Continentales - Continentale Verzekeringen NV
Organization And Summary Of Significant Accounting Policies [Line Items]
Business acquisition, percentage of ownership interest acquired				100.00%
Syndicate 1221
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage control of stamp capacity		100.00%