Statutory Accounting Principles - Additional Information (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statutory Accounting Practices [Line Items]
Cash dividends				$ 52,100,000		$ 36,500,000
Argo Re [Member]
Statutory Accounting Practices [Line Items]
Maximum permissible limit for dividend	25.00%		25.00%
Reduction in total statutory capital			15.00%
Maximum permitted amount of dividends				365,200,000
Cash dividends				52,100,000	$ 0	$ 36,500,000
Rockwood Casualty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Maximum permitted amount of dividends			$ 13,000,000.0
Cash dividends	$ 30,000,000.0	$ 20,000,000.0
Argonaut Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Maximum permitted amount of dividends			110,200,000
Cash dividends	50,000,000.0
Minimum [Member]
Statutory Accounting Practices [Line Items]
Minimum statutory capital and surplus balance	$ 242,900,000	$ 320,400,000	$ 242,900,000	$ 320,400,000