Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Mar. 08, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ (6,950)	$ (11,976)	$ (23,043)	$ (27,490)
Global Indemnity Group Inc | American Reliable Insurance Company
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve Settlement	$ 41,500
Proceeds for loss and loss adjustment expenses paid	38,800
Proceeds for accrued interest	6,200
Payment for the difference between the agreed upon purchase price and actual settlement	$ 3,500
Commercial Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(5,200)	(1,200)	(12,100)	(9,100)
Commercial Specialty | General Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(4,400)		(9,600)
Commercial Specialty | General Liability | Accident Years 2002 through 2004, 2006 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(3,100)
Commercial Specialty | General Liability | Construction Defect | Accident Years 2004 through 2012 and 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(500)		(1,000)
Commercial Specialty | General Liability | Other General Liability | Accident Years 1999 through 2014, 2016 and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(3,900)
Commercial Specialty | General Liability | Other General Liability | Accident Years 1999 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(8,600)
Commercial Specialty | Commercial Auto Liability
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,100)
Commercial Specialty | Commercial Auto Liability | Accident Years 2012, 2013 and 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(600)
Commercial Specialty | Commercial Auto Liability | Accident Year 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,500)
Commercial Specialty | Commercial Auto Liability | Accident Year 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			400
Commercial Specialty | Commercial Auto Liability | Accident Years 2000 through 2002, 2010, 2012 and 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,400)
Commercial Specialty | Commercial Auto Liability | Accident Years 2010, 2012, 2013 and 2015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(3,300)
Commercial Specialty | Property Lines | Accident Years 2014 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(1,900)
Commercial Specialty | Property Lines | Accident Years 2010 and 2012 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(900)
Commercial Specialty | Professional Liability | Accident Years 2009 and 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,100)
Commercial Specialty | Professional Liability | Accident Years 2010 through 2011 and 2014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(800)
Commercial Specialty | Reinsurance
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				1,000
Specialty Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,300)	(5,200)	(10,500)	(7,000)
Specialty Property | General Liability | Accident Years 2010 and 2014 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(400)
Specialty Property | General Liability | Accident Years 2011 through 2015 and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(1,500)
Specialty Property | General Liability | Accident Years 2010, 2015, 2016, and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(400)
Specialty Property | General Liability | Accident Years 2011 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(1,500)
Specialty Property | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(900)		(10,100)
Specialty Property | Property Lines | Accident Years 2018 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(600)		(8,900)
Specialty Property | Property Lines | Accident Year 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(200)
Specialty Property | Property Lines | Accident Years 2014 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(3,700)		(5,500)
Specialty Property | Property Lines | Accident Years 2016 and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,200)
Reinsurance Operations
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		700	(3,300)	3,500	(7,800)
Reinsurance Operations | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				3,800
Reinsurance Operations | Property Lines | Accident Years 2011, 2012, 2014 through 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		700
Reinsurance Operations | Property Lines | Accident Years 2007, 2009 through 2012 and 2014 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(3,300)
Reinsurance Operations | Property Lines | Accident Years 2010 Through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(4,100)
Reinsurance Operations | Property Lines | Accident Years 2007 and 2009 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					(7,800)
Reinsurance Operations | Property Lines | Typhoon Jebi | Accident Years 2018
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				7,600
Reinsurance Operations | Professional Liability | Accident Years 2007, 2008 and 2010
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(300)
Farm, Ranch & Stable
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		(1,200)	(2,300)	(4,000)	(3,600)
Farm, Ranch & Stable | Property Lines
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(2,200)
Farm, Ranch & Stable | Property Lines | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(1,100)
Farm, Ranch & Stable | Property Lines | Accident Years 2018 | Catastrophe
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve		$ (1,100)		(2,000)
Farm, Ranch & Stable | Property Lines | Accident Years 2014, 2016 and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			(400)
Farm, Ranch & Stable | Property Lines | Accident Years 2013 and 2015 Through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				(900)	(1,400)
Farm, Ranch & Stable | Liability | Accident Years 2014 through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve			$ (1,800)
Farm, Ranch & Stable | Liability | Accident Years 2013 Through 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve				$ (1,700)
Farm, Ranch & Stable | Liability | Accident Years 2007, 2012 Thought 2014, 2016, and 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Changes in prior year reserve					$ (2,200)