Organization and Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($) Insurance	Mar. 31, 2017 USD ($)	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	$ (33,655,000)	$ 51,148,000
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		$ 14,700,000
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	$ 7,500,000
Syndicate 1221
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage control of stamp capacity	100.00%