Significant business acquisitions - Estimated fair values of assets acquired and liabilities assumed (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 01, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Goodwill	$ 62,799		$ 60,714		$ 57,011
AltaLink [Member]
Business Acquisition [Line Items]
Cash and investments				$ 15
Property, plant and equipment				5,610
Goodwill				1,731
Other assets				287
Assets acquired				7,643
Accounts payable, accruals and other liabilities				1,064
Notes payable and other borrowings				3,851
Liabilities assumed				4,915
Net assets				$ 2,728
Berkshire Hathaway Automotive (formerly Van Tuyl Group) [Member]
Business Acquisition [Line Items]
Cash and investments		$ 1,274
Inventories		1,209
Property, plant and equipment		1,034
Goodwill		1,715
Other assets		1,400
Assets acquired		6,632
Accounts payable, accruals and other liabilities		1,210
Notes payable and other borrowings		1,129
Noncontrolling interests		181
Liabilities assumed and noncontrolling interests		2,520
Net assets		$ 4,112