Segments (Details) $ in Millions			3 Months Ended			6 Months Ended
		Apr. 01, 2021	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)		Jun. 30, 2021 USD ($) Subsidiaries Segment		Jun. 30, 2020 USD ($)
Segments [Abstract]
Number of reportable segments | Segment						4
Number of individual operating subsidiaries | Subsidiaries						3
Revenues [Abstract]
Insurance premiums			$ 2,035	$ 1,850		$ 3,997		$ 3,719
Net investment income (loss)			616	644		1,166		807
Investment gains (losses)	[1]		578	(1,142)		635		(1,358)
Non-insurance warranty revenue			359	308		697		609
Operating revenues and other			415	650		1,130		1,632
Total			4,003	2,310		7,625		5,409
Expenses [Abstract]
Insurance claims and policyholders' benefits			1,546	1,642		3,052		3,067
Amortization of deferred acquisition costs			357	342		716		686
Non-insurance warranty expense			332	285		643		566
Operating expenses and other			656	992		1,570		3,018
Interest			100	123		225		267
Total			2,991	3,384		6,206		7,604
Income (loss) before income tax			1,012	(1,074)		1,419		(2,195)
Income tax (expense) benefit			(219)	228		(333)		305
Net income (loss)			793	(846)		1,086		(1,890)
Amounts attributable to noncontrolling interests			(39)	11		(71)		423
Net income (loss) attributable to Loews Corporation			754	(835)		1,015		(1,467)
Altium Packaging LLC [Member]
Segments [Abstract]
Percentage interest of subsidiary sold		47.00%
Operating Segments [Member]
Revenues [Abstract]
Insurance premiums			2,035	1,850		3,997		3,719
Net investment income (loss)			616	644		1,166		807
Investment gains (losses)			578	(1,142)		635		(1,358)
Non-insurance warranty revenue			359	308		697		609
Operating revenues and other			415	650		1,130		1,632
Total			4,003	2,310		7,625		5,409
Expenses [Abstract]
Insurance claims and policyholders' benefits			1,546	1,642		3,052		3,067
Amortization of deferred acquisition costs			357	342		716		686
Non-insurance warranty expense			332	285		643		566
Operating expenses and other			656	992		1,570		3,018
Interest			100	123		225		267
Total			2,991	3,384		6,206		7,604
Income (loss) before income tax			1,012	(1,074)		1,419		(2,195)
Income tax (expense) benefit			(219)	228		(333)		305
Net income (loss)			793	(846)		1,086		(1,890)
Amounts attributable to noncontrolling interests			(39)	11		(71)		423
Net income (loss) attributable to Loews Corporation			754	(835)		1,015		(1,467)
Operating Segments [Member] | CNA Financial [Member]
Revenues [Abstract]
Insurance premiums			2,035	1,850		3,997		3,719
Net investment income (loss)			591	534		1,095		863
Investment gains (losses)			38	69		95		(147)
Non-insurance warranty revenue			359	308		697		609
Operating revenues and other			6	5		11		13
Total			3,029	2,766		5,895		5,057
Expenses [Abstract]
Insurance claims and policyholders' benefits			1,546	1,642		3,052		3,067
Amortization of deferred acquisition costs			357	342		716		686
Non-insurance warranty expense			332	285		643		566
Operating expenses and other			302	283		587		583
Interest			29	31		57		62
Total			2,566	2,583		5,055		4,964
Income (loss) before income tax			463	183		840		93
Income tax (expense) benefit			(94)	(32)		(160)		(4)
Net income (loss)			369	151		680		89
Amounts attributable to noncontrolling interests			(39)	(16)		(71)		(9)
Net income (loss) attributable to Loews Corporation			330	135		609		80
Operating Segments [Member] | Boardwalk Pipelines [Member]
Revenues [Abstract]
Operating revenues and other			312	296		684		637
Total			312	296		684		637
Expenses [Abstract]
Operating expenses and other			209	210		426		421
Interest			40	41		81		83
Total			249	251		507		504
Income (loss) before income tax			63	45		177		133
Income tax (expense) benefit			(16)	(11)		(45)		(34)
Net income (loss)			47	34		132		99
Net income (loss) attributable to Loews Corporation			47	34		132		99
Operating Segments [Member] | Loews Hotels & Co [Member]
Revenues [Abstract]
Net investment income (loss)			1			1	[2]
Operating revenues and other			97	34		154		176
Total			98	34		155		176
Expenses [Abstract]
Operating expenses and other			115	123		219		290
Interest			9	8		17		16
Total			124	131		236		306
Income (loss) before income tax			(26)	(97)		(81)		(130)
Income tax (expense) benefit			5	25		17		33
Net income (loss)			(21)	(72)		(64)		(97)
Net income (loss) attributable to Loews Corporation			(21)	(72)		(64)		(97)
Operating Segments [Member] | Corporate [Member]
Revenues [Abstract]
Net investment income (loss)			24	110	[3]	70	[2]	(56)	[3]
Investment gains (losses)			540	(1,211)	[3]	540	[2]	(1,211)	[3]
Operating revenues and other				244	[3]	281	[2]	501	[3]
Total			564	(857)	[3]	891	[2]	(766)	[3]
Expenses [Abstract]
Operating expenses and other			30	260	[3]	338	[2]	528	[3]
Interest			22	32	[3]	70	[2]	63	[3]
Total			52	292	[3]	408	[2]	591	[3]
Income (loss) before income tax			512	(1,149)	[3]	483	[2]	(1,357)	[3]
Income tax (expense) benefit			(114)	241	[3]	(145)	[2]	284	[3]
Net income (loss)			398	(908)	[3]	338	[2]	(1,073)	[3]
Net income (loss) attributable to Loews Corporation			$ 398	(908)	[3]	$ 338	[2]	(1,073)	[3]
Operating Segments [Member] | Diamond Offshore [Member]
Revenues [Abstract]
Operating revenues and other	[4]			71				305
Total	[4]			71				305
Expenses [Abstract]
Operating expenses and other	[4]			116				1,196
Interest	[4]			11				43
Total	[4]			127				1,239
Income (loss) before income tax	[4]			(56)				(934)
Income tax (expense) benefit	[4]			5				26
Net income (loss)	[4]			(51)				(908)
Amounts attributable to noncontrolling interests	[4]			27				432
Net income (loss) attributable to Loews Corporation	[4]			$ (24)				$ (476)

[1] Gross investment gains on available-for-sale securities were $51 and $102 for the three months ended June 30, 2021 and 2020 and $109 and $131 for the six months ended June 30, 2021 and 2020. Gross investment losses on available-for-sale securities were $20 and $85 for the three months ended June 30, 2021 and 2020 and $40 and $189 for the six months ended June 30, 2021 and 2020. During the three and six months ended June 30, 2021, $15 of investment gains were recognized due to the change in fair value of non-redeemable preferred stock still held as of June 30, 2021. During the three and six months ended June 30, 2020, $63 of investment gains and $70 of investment losses were recognized due to the change in fair value of non-redeemable preferred stock still held as of June 30, 2020.

[2] Amounts presented for Corporate include the operating results of Altium Packaging through March 31, 2021. Beginning April 1, 2021, Altium Packaging is recorded as an equity method investment.

[3] Amounts presented for Corporate include the operating results of Altium Packaging prior to the deconsolidation.

[4] Amounts presented for Diamond Offshore reflect the periods prior to the deconsolidation.