Quarterly Financial Data - Schedule of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Line Items]
Net loss	$ (165)	$ 278	$ 230	$ 293	$ 481	$ 157	$ 231	$ 295	$ 636	$ 1,164	$ 654
Net investment losses									$ (57)	122	$ 50
Non-cash increase in net income					$ 200					$ 200
CNA Financial [Member] | Catastrophe [Member]
Quarterly Financial Data [Line Items]
Net loss	(96)
Loews [Member]
Quarterly Financial Data [Line Items]
Net investment losses	(57)
Limited Partnership and Common Stock Investments [Member ]
Quarterly Financial Data [Line Items]
Net loss	$ (97)