Investments in equity securities - Narrative (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 2,980,000,000	$ 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	204,000,000	$ 65,000,000
International Business Machines Corporation [Member] | Common Stock [Member]
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 2,000,000,000
Unrealized losses on investments, percentage of cost	15.00%