Organization and Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Dec. 31, 2018 USD ($) Insurance	Dec. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Insurance	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)	Jun. 07, 2018	Jan. 01, 2018 USD ($)
Organization And Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in net cash used in investing activities			$ (260,147,000)	$ (264,029,000)	$ (202,183,000)
Impairment of goodwill and intangible assets			0	0	$ 0
Goodwill and intangible assets	$ 27,146,000	$ 6,596,000	27,146,000	$ 6,596,000
Changes in carrying value of goodwill and intangible assets			$ 20,500,000
Corporate income tax rate			21.00%	35.00%	35.00%
Transition tax		2,600,000		$ 2,600,000
Tax benefit (expense) related to transition tax			$ 1,300,000
Tax benefit (expense) related to remeasurement of net deferred tax assets	$ 500,000
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 (decrease) in net cash used in investing activities				14,400,000	$ (3,300,000)
Reclassification of restricted and unrestricted cash and cash equivalent		71,300,000		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
Minimum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life			3 years
Minimum | Accounting Standards Update 2016-02 | Subsequent Event
Organization And Summary Of Significant Accounting Policies [Line Items]
Additional operating liabilities						$ 45,000,000
Operating lease, Right-of-use asset						35,000,000
Maximum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life			7 years
Maximum | Accounting Standards Update 2016-02 | Subsequent Event
Organization And Summary Of Significant Accounting Policies [Line Items]
Additional operating liabilities						55,000,000
Operating lease, Right-of-use asset						$ 45,000,000
Leasehold Improvements | Minimum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life			5 years
Leasehold Improvements | Maximum
Organization And Summary Of Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life			10 years
Immaterial Error Correction
Organization And Summary Of Significant Accounting Policies [Line Items]
Cash reclassified from short- term investment to cash and cash equivalent		20,500,000		20,500,000
Overseas deposits reclassified from short-term investments to other invested assets		$ 28,800,000		28,800,000
Increase (decrease) in net cash used in investing activities				$ 1,900,000	$ (6,000,000)
Increase in income tax expense			$ 7,000,000
Quantifying misstatement in current year financial statements, amount			300,000
Immaterial Error Correction | Impacting Income Tax Expense Line Item
Organization And Summary Of Significant Accounting Policies [Line Items]
Quantifying misstatement in current year financial statements, amount			5,400,000
Immaterial Error Correction | Impacting Various Line Items
Organization And Summary Of Significant Accounting Policies [Line Items]
Quantifying misstatement in current year financial statements, amount			$ 5,100,000
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%
United States
Organization And Summary Of Significant Accounting Policies [Line Items]
Number of insurance companies | Insurance	2		2
Syndicate 1221
Organization And Summary Of Significant Accounting Policies [Line Items]
Percentage control of stamp capacity	100.00%		100.00%