Significant business acquisitions - Estimated fair values of assets acquired and liabilities assumed (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 01, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 62,708		$ 60,714		$ 57,011
AltaLink [Member]
Business Acquisition [Line Items]
Cash and investments				$ 15
Property, plant and equipment				5,610
Goodwill				1,744
Other assets				300
Assets acquired				7,669
Accounts payable, accruals and other liabilities				1,090
Notes payable and other borrowings				3,851
Liabilities assumed				4,941
Net assets				$ 2,728
Berkshire Hathaway Automotive (formerly Van Tuyl Group) [Member]
Business Acquisition [Line Items]
Cash and investments		$ 1,274
Inventories		1,219
Property, plant and equipment		1,045
Goodwill		1,833
Other assets		1,269
Assets acquired		6,640
Accounts payable, accruals and other liabilities		1,399
Notes payable and other borrowings		1,129
Liabilities assumed		2,528
Net assets		$ 4,112