Investments in equity securities - Narrative (Detail) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 3,260,000,000	$ 1,037,000,000
Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Other than temporary unrealized losses	0	0
Unrealized losses related to securities that have been in an unrealized loss position for more than 12 months	989,000,000	$ 65,000,000
International Business Machines Corporation [Member] | Common Stock [Member]
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 2,600,000,000
Unrealized losses on investments, percentage of cost	19.00%
Unrealized losses on investments, percentage related to market price decline in second half of 2015	77.00%