Segments - Consolidating Statement of Income and Total Assets by Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Insurance premiums	$ 1,824	$ 1,815	$ 3,627	$ 3,600
Net investment income	551	551	1,208	1,057
Investment gains (losses)	2	(3)	33	6
Non-insurance warranty revenue	285	248	566	486
Operating revenues and other	961	979	1,946	2,022
Total	3,623	3,590	7,380	7,171
Expenses:
Insurance claims and policyholders' benefits	1,352	1,327	2,709	2,666
Amortization of deferred acquisition costs	338	359	680	655
Non-insurance warranty expense	263	225	523	441
Operating expenses and other	1,231	1,229	2,380	2,413
Interest	164	143	305	284
Total	3,348	3,283	6,597	6,459
Income (loss) before income tax	275	307	783	712
Income tax (expense) benefit	(50)	(59)	(162)	(84)
Net income (loss)	225	248	621	628
Amounts attributable to noncontrolling interests	24	(18)	22	(105)
Net income (loss) attributable to Loews Corporation	249	230	643	523
CNA Financial [Member] | Operating Segments [Member]
Revenues:
Insurance premiums	1,824	1,815	3,627	3,600
Net investment income	515	506	1,086	996
Investment gains (losses)	2	(3)	33	6
Non-insurance warranty revenue	285	248	566	486
Operating revenues and other	4	8	13	21
Total	2,630	2,574	5,325	5,109
Expenses:
Insurance claims and policyholders' benefits	1,352	1,327	2,709	2,666
Amortization of deferred acquisition costs	338	359	680	655
Non-insurance warranty expense	263	225	523	441
Operating expenses and other	279	299	563	601
Interest	55	35	89	70
Total	2,287	2,245	4,564	4,433
Income (loss) before income tax	343	329	761	676
Income tax (expense) benefit	(64)	(60)	(141)	(115)
Net income (loss)	279	269	620	561
Amounts attributable to noncontrolling interests	(30)	(29)	(66)	(60)
Net income (loss) attributable to Loews Corporation	249	240	554	501
Diamond Offshore [Member] | Operating Segments [Member]
Revenues:
Net investment income	2	2	4	4
Operating revenues and other	222	269	456	566
Total	224	271	460	570
Expenses:
Operating expenses and other	335	320	618	616
Interest	31	30	61	58
Total	366	350	679	674
Income (loss) before income tax	(142)	(79)	(219)	(104)
Income tax (expense) benefit	36	10	42	54
Net income (loss)	(106)	(69)	(177)	(50)
Amounts attributable to noncontrolling interests	54	32	88	23
Net income (loss) attributable to Loews Corporation	(52)	(37)	(89)	(27)
Boardwalk Pipeline [Member] | Operating Segments [Member]
Revenues:
Operating revenues and other	327	285	673	622
Total	327	285	673	622
Expenses:
Operating expenses and other	209	203	404	401
Interest	46	43	91	87
Total	255	246	495	488
Income (loss) before income tax	72	39	178	134
Income tax (expense) benefit	(19)	(2)	(46)	(14)
Net income (loss)	53	37	132	120
Amounts attributable to noncontrolling interests		(21)		(68)
Net income (loss) attributable to Loews Corporation	53	16	132	52
Loews Hotels And Co [Member] | Operating Segments [Member]
Revenues:
Net investment income	1	1	1	1
Operating revenues and other	185	200	365	383
Total	186	201	366	384
Expenses:
Operating expenses and other	163	169	319	325
Interest	5	8	10	15
Total	168	177	329	340
Income (loss) before income tax	18	24	37	44
Income tax (expense) benefit	(6)	(7)	(12)	(14)
Net income (loss)	12	17	25	30
Net income (loss) attributable to Loews Corporation	12	17	25	30
Corporate [Member]
Revenues:
Net investment income	33	42	117	56
Operating revenues and other	223	217	439	430
Total	256	259	556	486
Expenses:
Operating expenses and other	245	238	476	470
Interest	27	27	54	54
Total	272	265	530	524
Income (loss) before income tax	(16)	(6)	26	(38)
Income tax (expense) benefit	3		(5)	5
Net income (loss)	(13)	(6)	21	(33)
Net income (loss) attributable to Loews Corporation	$ (13)	$ (6)	$ 21	$ (33)