Quarterly Financial Data - Schedule of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Line Items]
Net investment income	$ 116								$ 2,355	$ 1,817	$ 2,182
Asset impairment charges	69
Net loss	$ 217	$ 72	$ 249	$ 394	$ (165)	$ 278	$ 230	$ 293	932	636	1,164
Net investment losses									$ 49	$ (57)	$ 122
Active life reserve premium deficiency [Member]
Quarterly Financial Data [Line Items]
Net loss		$ 151
CNA Financial Corporation [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