Significant business acquisitions - Summary of preliminary values of identified assets acquired and liabilities assumed (Detail) - USD ($) $ in Millions	Dec. 31, 2023	[1]	Jan. 31, 2023	Dec. 31, 2022	[1]	Oct. 19, 2022	Dec. 31, 2021
Assets acquired
Goodwill	$ 84,626			$ 78,119			$ 73,875
Pilot Travel Centers LLC [Member]
Assets acquired
Property, plant and equipment			$ 8,015
Goodwill			6,605
Other intangible assets			6,853
Other assets			7,047
Assets acquired			28,520
Net assets			14,365
Liabilities assumed and noncontrolling interests
Notes payable			5,876
Other liabilities			4,918
Liabilities assumed			10,794
Noncontrolling interests, predominantly redeemable			3,361
Liabilities assumed and noncontrolling interests			$ 14,155
Alleghany Corporation [Member]
Assets acquired
Cash, cash equivalents and U.S. Treasury Bills						$ 3,762
Investments in fixed maturity and equity securities						15,982
Loans, receivables and other assets						9,287
Goodwill						3,900
Other intangible assets						2,659
Assets acquired						35,590
Net assets						11,505
Liabilities assumed and noncontrolling interests
Unpaid losses and loss adjustment expenses						15,080
Unearned premiums						3,536
Notes payable						2,169
Other liabilities						3,300
Liabilities assumed						$ 24,085

[1] Net of accumulated goodwill impairments of $11.1 billion as of December 31, 2023 and $11.0 billion as of December 31, 2022.