Segments (Details) $ in Millions		3 Months Ended
		Mar. 31, 2021 USD ($) Segment Subsidiaries	Mar. 31, 2020 USD ($)
Segments [Abstract]
Number of reportable segments | Segment		4
Number of individual operating subsidiaries | Subsidiaries		3
Revenues [Abstract]
Insurance premiums		$ 1,962	$ 1,869
Net investment income (loss)		550	163
Investment gains (losses)	[1]	57	(216)
Non-insurance warranty revenue		338	301
Operating revenues and other		715	982
Total		3,622	3,099
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,506	1,425
Amortization of deferred acquisition costs		359	344
Non-insurance warranty expense		311	281
Operating expenses and other		914	2,026
Interest		125	144
Total		3,215	4,220
Income (loss) before income tax		407	(1,121)
Income tax (expense) benefit		(114)	77
Net income (loss)		293	(1,044)
Amounts attributable to noncontrolling interests		(32)	412
Net income (loss) attributable to Loews Corporation		261	(632)
Operating Segments [Member]
Revenues [Abstract]
Insurance premiums		1,962	1,869
Net investment income (loss)		550	163
Investment gains (losses)		57	(216)
Non-insurance warranty revenue		338	301
Operating revenues and other		715	982
Total		3,622	3,099
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,506	1,425
Amortization of deferred acquisition costs		359	344
Non-insurance warranty expense		311	281
Operating expenses and other		914	2,026
Interest		125	144
Total		3,215	4,220
Income (loss) before income tax		407	(1,121)
Income tax (expense) benefit		(114)	77
Net income (loss)		293	(1,044)
Amounts attributable to noncontrolling interests		(32)	412
Net income (loss) attributable to Loews Corporation		261	(632)
Operating Segments [Member] | CNA Financial [Member]
Revenues [Abstract]
Insurance premiums		1,962	1,869
Net investment income (loss)		504	329
Investment gains (losses)		57	(216)
Non-insurance warranty revenue		338	301
Operating revenues and other		5	8
Total		2,866	2,291
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,506	1,425
Amortization of deferred acquisition costs		359	344
Non-insurance warranty expense		311	281
Operating expenses and other		285	300
Interest		28	31
Total		2,489	2,381
Income (loss) before income tax		377	(90)
Income tax (expense) benefit		(66)	28
Net income (loss)		311	(62)
Amounts attributable to noncontrolling interests		(32)	7
Net income (loss) attributable to Loews Corporation		279	(55)
Operating Segments [Member] | Boardwalk Pipelines [Member]
Revenues [Abstract]
Operating revenues and other		372	341
Total		372	341
Expenses [Abstract]
Operating expenses and other		217	211
Interest		41	42
Total		258	253
Income (loss) before income tax		114	88
Income tax (expense) benefit		(29)	(23)
Net income (loss)		85	65
Net income (loss) attributable to Loews Corporation		85	65
Operating Segments [Member] | Loews Hotels & Co [Member]
Revenues [Abstract]
Operating revenues and other		57	142
Total		57	142
Expenses [Abstract]
Operating expenses and other		104	167
Interest		8	8
Total		112	175
Income (loss) before income tax		(55)	(33)
Income tax (expense) benefit		12	8
Net income (loss)		(43)	(25)
Net income (loss) attributable to Loews Corporation		(43)	(25)
Operating Segments [Member] | Corporate [Member]
Revenues [Abstract]
Net investment income (loss)		46	(166)
Operating revenues and other		281	257
Total		327	91
Expenses [Abstract]
Operating expenses and other		308	268
Interest		48	31
Total		356	299
Income (loss) before income tax		(29)	(208)
Income tax (expense) benefit		(31)	43
Net income (loss)		(60)	(165)
Net income (loss) attributable to Loews Corporation		$ (60)	(165)
Operating Segments [Member] | Diamond Offshore [Member]
Revenues [Abstract]
Operating revenues and other			234
Total			234
Expenses [Abstract]
Operating expenses and other			1,080
Interest			32
Total			1,112
Income (loss) before income tax			(878)
Income tax (expense) benefit			21
Net income (loss)			(857)
Amounts attributable to noncontrolling interests			405
Net income (loss) attributable to Loews Corporation			$ (452)

[1] Gross investment gains on available-for-sale securities were $58 and $29 for the three months ended March 31, 2021 and 2020. Gross investment losses on available-for-sale securities were $20 and $104 for the three months ended March 31, 2021 and 2020. During the three months ended March 31, 2021 and 2020, $2 of investment gains and $133 of investment losses were recognized due to the change in fair value of non-redeemable preferred stock still held as of March 31, 2021 and 2020.