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Basis of presentation - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Other assets	$ 3,301		$ 2,443
Other liabilities	$ 4,987		4,505
cumulative effect adjustment on other assets and other liabilities		$ 642
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%
Subsequent to Adoption of Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Other assets		3,100	2,400
Other liabilities		$ 5,100	$ 4,500

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