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Significant business acquisitions - Narrative (Detail) - USD ($) $ in Millions				12 Months Ended
	Oct. 01, 2018	Feb. 29, 2016	Jan. 29, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Fair value of shares exchanged						$ 4,239
Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Aggregate consideration paid for acquisitions				$ 1,000	$ 2,700	$ 1,400
Precision Castparts Corp. [Member]
Business Acquisition [Line Items]
Acquisition price			$ 32,700
Assets			47,899
Cash			250
Liabilities			$ 15,241
MLMIC Insurance Company [Member]
Business Acquisition [Line Items]
Premiums earned				$ 400
MLMIC Insurance Company [Member] \| NICO [Member]
Business Acquisition [Line Items]
Acquisition price	$ 2,500
Assets	6,100
Cash	230
Investments	5,200
Liabilities	3,600
Unpaid losses and loss adjustment expenses	$ 3,200
Duracell Company [Member]
Business Acquisition [Line Items]
Assets		$ 5,143
Cash		1,807
Liabilities		904
Procter & Gamble Company [Member] \| Duracell Company [Member]
Business Acquisition [Line Items]
Fair value of shares exchanged		$ 4,239