Business Segments - Consolidated Revenues and Income (Loss) by Business Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Revenues	$ 3,333	$ 3,169	$ 3,435	$ 3,478	$ 3,521	$ 3,523	$ 3,593	$ 3,688	$ 13,415	$ 14,325	$ 14,613
Income (loss) before income tax and noncontrolling interests									244	1,810	2,277
Income from continuing operations					215	179	303	265	260	962	1,149
Discontinued operations, net					(7)	29	(187)	(206)		(371)	(554)
Net income (loss)	$ (201)	$ 182	$ 170	$ 109	$ 208	$ 208	$ 116	$ 59	260	591	595
Income Taxes									(43)	457	656
Interest Expense									520	498	425
CNA Financial [Member]
Segment Reporting Information [Line Items]
Revenues									9,101	9,692	9,932
Income (loss) before income tax and noncontrolling interests									553	1,215	1,283
Net income (loss)									433	802	827
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests									(71)	54	16
Investment gains (losses) included in net income (loss)									(34)	32	10
Income Taxes									71	322	363
Interest Expense									155	183	166
CNA Financial [Member] | Other Non-Core [Member]
Segment Reporting Information [Line Items]
Revenues									1,295	1,328	1,291
Income (loss) before income tax and noncontrolling interests									(830)	(331)	(501)
Net income (loss)									(387)	(123)	(230)
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests									8	24	31
Investment gains (losses) included in net income (loss)									12	15	18
Income Taxes									(397)	(195)	(245)
Interest Expense									154	182	165
CNA Financial [Member] | Property and Casualty [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Revenues									3,579	3,708	3,676
Income (loss) before income tax and noncontrolling interests									810	967	1,005
Net income (loss)									483	578	598
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests									(33)	15	(5)
Investment gains (losses) included in net income (loss)									(19)	9	(2)
Income Taxes									271	324	340
CNA Financial [Member] | Property and Casualty [Member] | Commercial [Member]
Segment Reporting Information [Line Items]
Revenues									3,371	3,683	3,984
Income (loss) before income tax and noncontrolling interests									514	477	662
Net income (loss)									303	285	394
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests									(47)	16	(15)
Investment gains (losses) included in net income (loss)									(28)	9	(9)
Income Taxes									175	159	223
CNA Financial [Member] | Property and Casualty [Member] | International [Member]
Segment Reporting Information [Line Items]
Revenues									856	973	981
Income (loss) before income tax and noncontrolling interests									59	102	117
Net income (loss)									34	62	65
Investment gains (losses) included in Revenues and Income (loss) before income tax and noncontrolling interests									1	(1)	5
Investment gains (losses) included in net income (loss)									1	(1)	3
Income Taxes									22	34	45
Interest Expense									1	1	1
Diamond Offshore [Member]
Segment Reporting Information [Line Items]
Revenues									2,428	2,825	2,926
Income (loss) before income tax and noncontrolling interests									(402)	514	774
Net income (loss)									(156)	183	257
Income Taxes									(117)	142	245
Interest Expense									94	62	25
Boardwalk Pipeline [Member]
Segment Reporting Information [Line Items]
Revenues									1,254	1,236	1,232
Income (loss) before income tax and noncontrolling interests									227	140	241
Net income (loss)									74	18	78
Income Taxes									46	11	56
Interest Expense									176	165	163
Loews Hotels [Member]
Segment Reporting Information [Line Items]
Revenues									604	475	380
Income (loss) before income tax and noncontrolling interests									28	21	(4)
Net income (loss)									12	11	(3)
Income Taxes									16	10	(1)
Interest Expense									21	14	9
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Revenues									28	97	143
Income (loss) before income tax and noncontrolling interests									(162)	(80)	(17)
Net income (loss)									(103)	(52)	(10)
Income Taxes									(59)	(28)	(7)
Interest Expense									$ 74	$ 74	$ 62