Investments in equity securities - Narrative (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 2,119,000,000	$ 3,260,000,000
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	722,000,000	$ 989,000,000
International Business Machines Corporation [Member] | Common Stock [Member]
Summary of Investment Holdings [Line Items]
Unrealized losses on investments	$ 1,500,000,000
Unrealized losses on investments, percentage of cost	11.00%