Business acquisitions - Narrative (Detail) - USD ($) $ in Millions		12 Months Ended
	Oct. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Aggregate consideration paid for acquisitions, net of cash acquired		$ 1,683	$ 3,279	$ 2,708
MLMIC Insurance Company [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
Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Aggregate consideration paid for acquisitions, net of cash acquired		$ 1,700	$ 1,000	$ 2,700