Business Segments - Consolidated Revenues and Income (Loss) by Business Segment (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Revenues	$ 3,287	$ 3,169	$ 9,767	$ 10,082
Income (loss) before income tax and noncontrolling interests	556	348	475	748
Net income (loss)	327	182	364	461
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	45	(50)	18	(42)
Investment gains (losses) included in net income (loss)	27	(29)	12	(18)
CNA Financial [Member]
Segment Reporting Information [Line Items]
Revenues	2,433	2,153	6,984	6,832
Income (loss) before income tax and noncontrolling interests	476	234	824	713
Net income (loss)	308	161	557	495
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	45	(50)	30	(42)
Investment gains (losses) included in net income (loss)	27	(29)	16	(18)
CNA Financial [Member] \| Other Non-Core [Member]
Segment Reporting Information [Line Items]
Revenues	345	324	996	978
Income (loss) before income tax and noncontrolling interests	(49)	(120)	(355)	(410)
Net income (loss)	(4)	(44)	(141)	(167)
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	18	2	12	(2)
Investment gains (losses) included in net income (loss)	11	1	4	5
CNA Financial [Member] \| Property and Casualty [Member] \| Specialty [Member]
Segment Reporting Information [Line Items]
Revenues	946	846	2,739	2,667
Income (loss) before income tax and noncontrolling interests	306	248	736	661
Net income (loss)	180	147	437	394
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	9	(22)	2	(18)
Investment gains (losses) included in net income (loss)	5	(14)	1	(11)
CNA Financial [Member] \| Property and Casualty [Member] \| Commercial [Member]
Segment Reporting Information [Line Items]
Revenues	913	767	2,591	2,545
Income (loss) before income tax and noncontrolling interests	186	83	427	391
Net income (loss)	110	49	252	231
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	12	(29)	2	(23)
Investment gains (losses) included in net income (loss)	7	(16)	1	(13)
CNA Financial [Member] \| Property and Casualty [Member] \| International [Member]
Segment Reporting Information [Line Items]
Revenues	229	216	658	642
Income (loss) before income tax and noncontrolling interests	33	23	16	71
Net income (loss)	22	9	9	37
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests	6	(1)	14	1
Investment gains (losses) included in net income (loss)	4		10	1
Diamond Offshore [Member]
Segment Reporting Information [Line Items]
Revenues	350	608	1,211	1,867
Income (loss) before income tax and noncontrolling interests	36	139	(538)	(42)
Net income (loss)	7	47	(240)	(34)
Boardwalk Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues	306	296	961	925
Income (loss) before income tax and noncontrolling interests	46	48	210	163
Net income (loss)	14	18	62	55
Loews Hotels [Member]
Segment Reporting Information [Line Items]
Revenues	161	146	513	452
Income (loss) before income tax and noncontrolling interests	4	1	17	25
Net income (loss)	3	2	7	15
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues	37	(34)	98	6
Income (loss) before income tax and noncontrolling interests	(6)	(74)	(38)	(111)
Net income (loss)	$ (5)	$ (46)	(22)	$ (70)
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests			(12)
Investment gains (losses) included in net income (loss)			$ (4)

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Investment Gains Losses Included In Revenue And Income Loss Before Income Tax And Noncontrolling Interests

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Amount of income (loss) from continuing operations, including income (loss) from equity method investments, before deduction of income tax expense (benefit), and income (loss) attributable to noncontrolling interest.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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