Segments (Details) $ in Millions		3 Months Ended																12 Months Ended
		Dec. 31, 2020 USD ($)		Sep. 30, 2020 USD ($)		Jun. 30, 2020 USD ($)		Mar. 31, 2020 USD ($)		Dec. 31, 2019 USD ($)		Sep. 30, 2019 USD ($)		Jun. 30, 2019 USD ($)		Mar. 31, 2019 USD ($)		Dec. 31, 2020 USD ($) Facility Hotel Segment Subsidiaries mi	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segments [Abstract]
Number of reportable segments | Segment																		5
Number of individual operating subsidiaries | Subsidiaries																		3
Number of states having natural gas storage facilities | Facility																		4
Number of miles of natural gas and natural gas liquid pipelines | mi																		14,095
Number of hotels | Hotel																		27
Revenues [Abstract]
Insurance premiums																		$ 7,649	$ 7,428	$ 7,312
Net investment income (loss)																		1,995	2,355	1,817
Investment gains (losses)	[1]																	(1,246)	49	(57)
Non-insurance warranty revenue																		1,252	1,161	1,007
Operating revenues and other																		2,933	3,938	3,987
Total		$ 3,709		$ 3,465		$ 2,310		$ 3,099		$ 3,876		$ 3,675		$ 3,623		$ 3,757		12,583	14,931	14,066
Expenses [Abstract]
Insurance claims and policyholders' benefits																		6,170	5,806	5,572
Amortization of deferred acquisition costs																		1,410	1,383	1,335
Non-insurance warranty expense																		1,159	1,082	923
Operating expenses and other																		4,793	4,950	4,828
Interest																		515	591	574
Total																		14,047	13,812	13,232
Income (loss) before income tax																		(1,464)	1,119	834
Income tax (expense) benefit																		173	(248)	(128)
Net income (loss)																		(1,291)	871	706
Amounts attributable to noncontrolling interests																		360	61	(70)
Net income (loss) attributable to Loews Corporation		397	[2],[3],[4]	$ 139	[2],[3],[4]	$ (835)	[2],[3],[4]	$ (632)	[2],[3],[4]	217	[5],[6]	$ 72	[5],[6]	$ 249	[5],[6]	$ 394	[5],[6]	(931)	932	636
Assets [Abstract]
Total assets		80,236								82,243								80,236	82,243
Operating Segments [Member]
Revenues [Abstract]
Insurance premiums																		7,649	7,428	7,312
Net investment income (loss)																		1,995	2,355	1,817
Investment gains (losses)																		(1,246)	49	(57)
Non-insurance warranty revenue																		1,252	1,161	1,007
Operating revenues and other																		2,933	3,938	3,987
Total																		12,583	14,931	14,066
Expenses [Abstract]
Insurance claims and policyholders' benefits																		6,170	5,806	5,572
Amortization of deferred acquisition costs																		1,410	1,383	1,335
Non-insurance warranty expense																		1,159	1,082	923
Operating expenses and other																		4,793	4,950	4,828
Interest																		515	591	574
Total																		14,047	13,812	13,232
Income (loss) before income tax																		(1,464)	1,119	834
Income tax (expense) benefit																		173	(248)	(128)
Net income (loss)																		(1,291)	871	706
Amounts attributable to noncontrolling interests																		360	61	(70)
Net income (loss) attributable to Loews Corporation																		(931)	932	636
Assets [Abstract]
Total assets		80,236								82,243								80,236	82,243
Operating Segments [Member] | CNA Financial [Member]
Revenues [Abstract]
Insurance premiums																		7,649	7,428	7,312
Net investment income (loss)																		1,935	2,118	1,817
Investment gains (losses)																		(35)	49	(57)
Non-insurance warranty revenue																		1,252	1,161	1,007
Operating revenues and other																		26	32	55
Total																		10,827	10,788	10,134
Expenses [Abstract]
Insurance claims and policyholders' benefits																		6,170	5,806	5,572
Amortization of deferred acquisition costs																		1,410	1,383	1,335
Non-insurance warranty expense																		1,159	1,082	923
Operating expenses and other																		1,125	1,141	1,203
Interest																		142	152	138
Total																		10,006	9,564	9,171
Income (loss) before income tax																		821	1,224	963
Income tax (expense) benefit																		(131)	(224)	(151)
Net income (loss)																		690	1,000	812
Amounts attributable to noncontrolling interests																		(72)	(106)	(86)
Net income (loss) attributable to Loews Corporation																		618	894	726
Assets [Abstract]
Total assets		63,976								60,583								63,976	60,583
Operating Segments [Member] | Boardwalk Pipelines [Member]
Revenues [Abstract]
Operating revenues and other																		1,302	1,300	1,227
Total																		1,302	1,300	1,227
Expenses [Abstract]
Operating expenses and other																		855	840	820
Interest																		170	179	176
Total																		1,025	1,019	996
Income (loss) before income tax																		277	281	231
Income tax (expense) benefit																		(71)	(72)	(28)
Net income (loss)																		206	209	203
Amounts attributable to noncontrolling interests																				(68)
Net income (loss) attributable to Loews Corporation																		206	209	135
Assets [Abstract]
Total assets		9,353								9,248								9,353	9,248
Operating Segments [Member] | Loews Hotels & Co [Member]
Revenues [Abstract]
Net investment income (loss)																		1	1	2
Operating revenues and other																		277	691	753
Total																		278	692	755
Expenses [Abstract]
Operating expenses and other																		519	698	653
Interest																		33	22	29
Total																		552	720	682
Income (loss) before income tax																		(274)	(28)	73
Income tax (expense) benefit																		62	(3)	(25)
Net income (loss)																		(212)	(31)	48
Net income (loss) attributable to Loews Corporation																		(212)	(31)	48
Assets [Abstract]
Total assets		1,637								1,728								1,637	1,728
Operating Segments [Member] | Corporate [Member]
Revenues [Abstract]
Net investment income (loss)																		59	230	(10)
Investment gains (losses)																		(1,211)
Operating revenues and other																		1,023	933	867
Total																		(129)	1,163	857
Expenses [Abstract]
Operating expenses and other																		1,098	1,004	956
Interest																		127	115	108
Total																		1,225	1,119	1,064
Income (loss) before income tax																		(1,354)	44	(207)
Income tax (expense) benefit																		287	(9)	46
Net income (loss)																		(1,067)	35	(161)
Net income (loss) attributable to Loews Corporation																		(1,067)	35	(161)
Assets [Abstract]
Total assets		5,270								4,850								5,270	4,850
Operating Segments [Member] | Diamond Offshore [Member]
Revenues [Abstract]
Net investment income (loss)	[7]																		6	8
Operating revenues and other	[7]																	305	982	1,085
Total	[7]																	305	988	1,093
Expenses [Abstract]
Operating expenses and other	[7]																	1,196	1,267	1,196
Interest	[7]																	43	123	123
Total	[7]																	1,239	1,390	1,319
Income (loss) before income tax	[7]																	(934)	(402)	(226)
Income tax (expense) benefit	[7]																	26	60	30
Net income (loss)	[7]																	(908)	(342)	(196)
Amounts attributable to noncontrolling interests	[7]																	432	167	84
Net income (loss) attributable to Loews Corporation	[7]																	(476)	(175)	$ (112)
Assets [Abstract]
Total assets	[7]	$ 0								$ 5,834								$ 0	$ 5,834
United States [Member] | Loews Hotels & Co [Member]
Segments [Abstract]
Number of hotels | Hotel																		26
Canada [Member] | Loews Hotels & Co [Member]
Segments [Abstract]
Number of hotels | Hotel																		1

[1] Gross investment gains on available-for-sale securities were $220, $125 and $168 for the years ended December 31, 2020, 2019 and 2018. Gross investment losses on available-for-sale securities were $220, $131 and $164 for the years ended December 31, 2020, 2019 and 2018. For the year ended December 31, 2020, $3 of investment losses were recognized due to the change in fair value of non-redeemable preferred stock still held as of December 31, 2020. For the year ended December 31, 2019, $66 of investment gains were recognized due to the change in fair value of non-redeemable preferred stock still held as of December 31, 2019.

[2] Net income for the third quarter of 2020 included net catastrophe losses of $112 million (after tax and noncontrolling interests) and a $52 million (after tax and noncontrolling interests) charge related to the recognition of an active life reserve premium deficiency as a result of the third quarter GPV.

[3] Net loss for the first quarter of 2020 included an aggregate asset impairment charge of $408 million (after tax and noncontrolling interests) related to Diamond Offshore drilling rigs.

[4] Net loss for the second quarter of 2020 included an investment loss of $957 million (after tax) related to the deconsolidation of Diamond Offshore and net catastrophe losses of $212 million (after tax and noncontrolling interests).

[5] Net income for the fourth quarter of 2019 included net investment income of $116 million (after tax and noncontrolling interests) related to limited partnerships and equity securities partially offset by asset impairment charges of $69 million (after tax).

[6] Net income for the third quarter of 2019 included a $151 million (after tax and noncontrolling interests) charge related to the recognition of an active life reserve premium deficiency as a result of the third quarter GPV.

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