Business Segments - Consolidated Revenues and Income (Loss) by Business Segment (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 3,307	$ 3,435	$ 6,480	$ 6,913
Income (loss) before income tax and noncontrolling interests	(276)	295	(81)	400
Net income (loss)	(65)	170	37	279
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	1	(2)	(27)	8
Investment gains (losses) included in net income (loss)	2	3	(15)	11
CNA Financial [Member]
Segment Reporting Information [Line Items]
Revenues	2,348	2,327	4,551	4,679
Income (loss) before income tax and noncontrolling interests	290	165	348	479
Net income (loss)	189	124	249	334
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	13	(2)	(15)	8
Investment gains (losses) included in net income (loss)	6	3	(11)	11
CNA Financial [Member] | Other Non-Core [Member]
Segment Reporting Information [Line Items]
Revenues	330	320	651	654
Income (loss) before income tax and noncontrolling interests	(79)	(198)	(306)	(290)
Net income (loss)	(26)	(91)	(137)	(123)
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	(3)	(5)	(6)	(4)
Investment gains (losses) included in net income (loss)	(4)	2	(7)	4
CNA Financial [Member] | Property and Casualty [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Revenues	928	904	1,793	1,821
Income (loss) before income tax and noncontrolling interests	250	206	430	413
Net income (loss)	150	124	257	247
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	4		(7)	4
Investment gains (losses) included in net income (loss)	3	1	(4)	3
CNA Financial [Member] | Property and Casualty [Member] | Commercial [Member]
Segment Reporting Information [Line Items]
Revenues	876	883	1,678	1,778
Income (loss) before income tax and noncontrolling interests	146	122	241	308
Net income (loss)	86	72	142	182
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	8	2	(10)	6
Investment gains (losses) included in net income (loss)	4		(6)	3
CNA Financial [Member] | Property and Casualty [Member] | International [Member]
Segment Reporting Information [Line Items]
Revenues	214	220	429	426
Income (loss) before income tax and noncontrolling interests	(27)	35	(17)	48
Net income (loss)	(21)	19	(13)	28
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	4	1	8	2
Investment gains (losses) included in net income (loss)	3		6	1
Diamond Offshore [Member]
Segment Reporting Information [Line Items]
Revenues	390	632	861	1,259
Income (loss) before income tax and noncontrolling interests	(657)	106	(574)	(181)
Net income (loss)	(290)	45	(247)	(81)
Boardwalk Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues	308	299	655	629
Income (loss) before income tax and noncontrolling interests	65	38	164	115
Net income (loss)	17	12	48	37
Loews Hotels [Member]
Segment Reporting Information [Line Items]
Revenues	189	167	352	306
Income (loss) before income tax and noncontrolling interests	4	14	13	24
Net income (loss)	1	8	4	13
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	72	10	61	40
Income (loss) before income tax and noncontrolling interests	22	(28)	(32)	(37)
Net income (loss)	18	$ (19)	(17)	$ (24)
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	(12)		(12)
Investment gains (losses) included in net income (loss)	$ (4)		$ (4)