Significant business acquisitions - Summary of preliminary values of identified assets acquired and liabilities assumed (Detail) - USD ($) $ in Millions	Jun. 30, 2023	[1]	Jan. 31, 2023	Dec. 31, 2022	[1]	Oct. 19, 2022	Dec. 31, 2021
Business Acquisition [Line Items]
Goodwill	$ 85,853			$ 78,119			$ 73,875
Pilot Travel Centers LLC [Member]
Business Acquisition [Line Items]
Property, plant and equipment			$ 8,082
Goodwill and other intangible assets			13,309
Other assets			6,994
Assets acquired			28,385
Notes payable			5,876
Other liabilities			4,774
Liabilities assumed			10,650
Noncontrolling interests, predominantly redeemable			3,370
Net assets			$ 14,365
Alleghany Corporation [Member]
Business Acquisition [Line Items]
Cash, cash equivalents and U.S. Treasury Bills						$ 3,762
Investments in fixed maturity and equity securities						15,982
Loans and other receivables						5,650
Other intangible assets						2,659
Goodwill						3,900
Other assets						3,637
Assets acquired						35,590
Unpaid losses and loss adjustment expenses						15,080
Unearned premiums						3,536
Notes payable						2,169
Other liabilities						3,300
Liabilities assumed						24,085
Net assets						$ 11,505

[1] Net of accumulated goodwill impairments of $11.0 billion as of June 30, 2023 and December 31, 2022.