Basis of presentation - Additional Information (Detail) - USD ($) $ in Millions			3 Months Ended			6 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Shares excluded from diluted EPS calculation			0		600,000	0	600,000
Other assets		$ 2,555	$ 4,791	$ 2,555		$ 4,791
Other liabilities		$ 5,466	9,146	5,466		9,146
Non-cash increase in net income				200
Increase to net income for uncertain tax positions						$ 23
Tax rate		35.00%				21.00%
CNA Financial [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage						89.00%
Diamond Offshore Drilling, Inc. [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage						53.00%
Consolidated Container [Member]
Summary Of Significant Accounting Policies [Line Items]
Subsidiary ownership percentage						99.00%
Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	$ (62)
Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	25
Accounting Standards Update 2016-16 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	(9)
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment	3
Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	(3)
Deferred Tax Asset [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	23
Other Assets [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	1,900
Other Liabilities [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	2,000
Pension Liability [Member] | Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	(130)
Pretax Loss on Investments [Member] | Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update			(8)			$ (23)
Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update					$ 2		$ 6
Unrealized Gain Loss on Investments [Member] | Accumulated Other Comprehensive Income (Loss) [Member] | Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update	$ 127
Operating Revenues and Other [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update			143			274
Operating Expenses and Other [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Changes due to adoption of new accounting standards update			141			274
Prior to Adoption of Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Other assets		$ 2,600	2,700	2,600		2,700
Other liabilities		$ 5,500	$ 7,000	$ 5,500		$ 7,000