Segments - Consolidating Statement of Income and Total Assets by Segment (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
Revenues:
Insurance premiums									$ 7,428	$ 7,312	$ 6,988
Net investment income (loss)	$ 116								2,355	1,817	2,182
Investment gains (losses)									49	(57)	122
Non-insurance warranty revenue									1,161	1,007	390
Operating revenues and other									3,938	3,987	4,053
Total	3,876	$ 3,675	$ 3,623	$ 3,757	$ 3,287	$ 3,608	$ 3,590	$ 3,581	14,931	14,066	13,735
Expenses:
Insurance claims and policyholders' benefits									5,806	5,572	5,310
Amortization of deferred acquisition costs									1,383	1,335	1,233
Non-insurance warranty expense									1,082	923	299
Operating expenses and other									4,950	4,828	4,665
Interest									591	574	646
Total									13,812	13,232	12,153
Income (loss) before income tax									1,119	834	1,582
Income tax (expense) benefit									(248)	(128)	(170)
Net income (loss)									871	706	1,412
Amounts attributable to noncontrolling interests									61	(70)	(248)
Net income (loss) attributable to Loews Corporation	217	$ 72	$ 249	$ 394	(165)	$ 278	$ 230	$ 293	932	636	1,164
Total assets	82,243				78,316				82,243	78,316
CNA Financial Corporation [Member] | Operating Segments [Member]
Revenues:
Insurance premiums									7,428	7,312	6,988
Net investment income (loss)									2,118	1,817	2,034
Investment gains (losses)									49	(57)	122
Non-insurance warranty revenue									1,161	1,007	390
Operating revenues and other									32	55	49
Total									10,788	10,134	9,583
Expenses:
Insurance claims and policyholders' benefits									5,806	5,572	5,310
Amortization of deferred acquisition costs									1,383	1,335	1,233
Non-insurance warranty expense									1,082	923	299
Operating expenses and other									1,141	1,203	1,224
Interest									152	138	203
Total									9,564	9,171	8,269
Income (loss) before income tax									1,224	963	1,314
Income tax (expense) benefit									(224)	(151)	(419)
Net income (loss)									1,000	812	895
Amounts attributable to noncontrolling interests									(106)	(86)	(94)
Net income (loss) attributable to Loews Corporation									894	726	801
Total assets	60,583				57,123				60,583	57,123
Diamond Offshore [Member] | Operating Segments [Member]
Revenues:
Net investment income (loss)									6	8	2
Operating revenues and other									982	1,085	1,498
Total									988	1,093	1,500
Expenses:
Operating expenses and other									1,267	1,196	1,373
Interest									123	123	149
Total									1,390	1,319	1,522
Income (loss) before income tax									(402)	(226)	(22)
Income tax (expense) benefit									60	30	4
Net income (loss)									(342)	(196)	(18)
Amounts attributable to noncontrolling interests									167	84	(9)
Net income (loss) attributable to Loews Corporation									(175)	(112)	(27)
Total assets	5,834				6,036				5,834	6,036
Boardwalk Pipeline [Member] | Operating Segments [Member]
Revenues:
Operating revenues and other									1,300	1,227	1,325
Total									1,300	1,227	1,325
Expenses:
Operating expenses and other									840	820	861
Interest									179	176	171
Total									1,019	996	1,032
Income (loss) before income tax									281	231	293
Income tax (expense) benefit									(72)	(28)	232
Net income (loss)									209	203	525
Amounts attributable to noncontrolling interests										(68)	(145)
Net income (loss) attributable to Loews Corporation									209	135	380
Total assets	9,248				9,131				9,248	9,131
Loews Hotels And Co [Member] | Operating Segments [Member]
Revenues:
Net investment income (loss)									1	2
Operating revenues and other									691	753	682
Total									692	755	682
Expenses:
Operating expenses and other									698	653	589
Interest									22	29	28
Total									720	682	617
Income (loss) before income tax									(28)	73	65
Income tax (expense) benefit									(3)	(25)	(1)
Net income (loss)									(31)	48	64
Net income (loss) attributable to Loews Corporation									(31)	48	64
Total assets	1,728				1,812				1,728	1,812
Corporate [Member]
Revenues:
Net investment income (loss)									230	(10)	146
Operating revenues and other									933	867	499
Total									1,163	857	645
Expenses:
Operating expenses and other									1,004	956	618
Interest									115	108	95
Total									1,119	1,064	713
Income (loss) before income tax									44	(207)	(68)
Income tax (expense) benefit									(9)	46	14
Net income (loss)									35	(161)	(54)
Net income (loss) attributable to Loews Corporation									35	(161)	$ (54)
Total assets	$ 4,850				$ 4,214				$ 4,850	$ 4,214