Organization and Summary of Significant Accounting Policies - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($) Insurance	Jun. 30, 2017 USD ($)	Jun. 07, 2018	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Number of insurance companies | Insurance	2
Corporate income tax rate	21.00%
Increase net cash used in investing activities	$ 8,191,000	$ 77,921,000
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]
Increase net cash used in investing activities		18,100,000
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
Increase net cash used in investing activities		$ 4,800,000
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%