Segments (Details) $ in Millions		3 Months Ended
		Mar. 31, 2020 USD ($) Subsidiaries Segment	Mar. 31, 2019 USD ($)
Segment [Abstract]
Number of reportable segments | Segment		5
Number of individual operating subsidiaries | Subsidiaries		4
Revenues [Abstract]
Insurance premiums		$ 1,869	$ 1,803
Net investment income (loss)		163	657
Investment gains (losses)	[1]	(216)	31
Non-insurance warranty revenue		301	281
Operating revenues and other		982	985
Total		3,099	3,757
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,425	1,357
Amortization of deferred acquisition costs		344	342
Non-insurance warranty expense		281	260
Operating expenses and other		2,026	1,149
Interest		144	141
Total		4,220	3,249
Income (loss) before income tax		(1,121)	508
Income tax (expense) benefit		77	(112)
Net income (loss)		(1,044)	396
Amounts attributable to noncontrolling interests		412	(2)
Net income (loss) attributable to Loews Corporation		(632)	394
Operating Segments [Member]
Revenues [Abstract]
Insurance premiums		1,869	1,803
Net investment income (loss)		163	657
Investment gains (losses)		(216)	31
Non-insurance warranty revenue		301	281
Operating revenues and other		982	985
Total		3,099	3,757
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,425	1,357
Amortization of deferred acquisition costs		344	342
Non-insurance warranty expense		281	260
Operating expenses and other		2,026	1,149
Interest		144	141
Total		4,220	3,249
Income (loss) before income tax		(1,121)	508
Income tax (expense) benefit		77	(112)
Net income (loss)		(1,044)	396
Amounts attributable to noncontrolling interests		412	(2)
Net income (loss) attributable to Loews Corporation		(632)	394
CNA Financial [Member] | Operating Segments [Member]
Revenues [Abstract]
Insurance premiums		1,869	1,803
Net investment income (loss)		329	571
Investment gains (losses)		(216)	31
Non-insurance warranty revenue		301	281
Operating revenues and other		8	9
Total		2,291	2,695
Expenses [Abstract]
Insurance claims and policyholders' benefits		1,425	1,357
Amortization of deferred acquisition costs		344	342
Non-insurance warranty expense		281	260
Operating expenses and other		300	284
Interest		31	34
Total		2,381	2,277
Income (loss) before income tax		(90)	418
Income tax (expense) benefit		28	(77)
Net income (loss)		(62)	341
Amounts attributable to noncontrolling interests		7	(36)
Net income (loss) attributable to Loews Corporation		(55)	305
Diamond Offshore [Member] | Operating Segments [Member]
Revenues [Abstract]
Net investment income (loss)			2
Operating revenues and other		234	234
Total		234	236
Expenses [Abstract]
Operating expenses and other		1,080	283
Interest		32	30
Total		1,112	313
Income (loss) before income tax		(878)	(77)
Income tax (expense) benefit		21	6
Net income (loss)		(857)	(71)
Amounts attributable to noncontrolling interests		405	34
Net income (loss) attributable to Loews Corporation		(452)	(37)
Boardwalk Pipeline [Member] | Operating Segments [Member]
Revenues [Abstract]
Operating revenues and other		341	346
Total		341	346
Expenses [Abstract]
Operating expenses and other		211	195
Interest		42	45
Total		253	240
Income (loss) before income tax		88	106
Income tax (expense) benefit		(23)	(27)
Net income (loss)		65	79
Net income (loss) attributable to Loews Corporation		65	79
Loews Hotels & Co [Member] | Operating Segments [Member]
Revenues [Abstract]
Operating revenues and other		142	180
Total		142	180
Expenses [Abstract]
Operating expenses and other		167	156
Interest		8	5
Total		175	161
Income (loss) before income tax		(33)	19
Income tax (expense) benefit		8	(6)
Net income (loss)		(25)	13
Net income (loss) attributable to Loews Corporation		(25)	13
Corporate [Member] | Operating Segments [Member]
Revenues [Abstract]
Net investment income (loss)		(166)	84
Operating revenues and other		257	216
Total		91	300
Expenses [Abstract]
Operating expenses and other		268	231
Interest		31	27
Total		299	258
Income (loss) before income tax		(208)	42
Income tax (expense) benefit		43	(8)
Net income (loss)		(165)	34
Net income (loss) attributable to Loews Corporation		$ (165)	$ 34

[1] Gross investment gains on available-for-sale securities were $29 and $36 for the three months ended March 31, 2020 and 2019. Gross investment losses on available-for-sale securities were $104 and $42 for the three months ended March 31, 2020 and 2019. During the three months ended March 31, 2020 and 2019, $133 of net investment losses and $42 of net investment gains were recognized due to the change in fair value of non-redeemable preferred stock still held as of March 31, 2020 and 2019.