Quarterly data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Revenues	$ 58,907		$ 60,525		$ 57,518		$ 65,187		$ 58,344		$ 58,843		$ 54,254		$ 52,163		$ 242,137	$ 223,604	$ 210,943
Net earnings attributable to Berkshire shareholders	32,551	[1]	4,067	[1]	4,262	[1]	4,060	[1]	6,286	[1]	7,198	[1]	5,001	[1]	5,589	[1]	44,940	$ 24,074	$ 24,083
Investment and derivative gains/losses	107		$ 623		$ 143		$ 504		$ 1,904		$ 2,347		$ 394		$ 1,852
Income tax net benefit - Tax Cuts and Jobs Act of 2017	$ 28,200																$ 28,200
Equivalent Class A [Member]
Quarterly Financial Information [Line Items]
Net earnings attributable to Berkshire shareholders per equivalent Class A common share	$ 19,790		$ 2,473		$ 2,592		$ 2,469		$ 3,823		$ 4,379		$ 3,042		$ 3,401		$ 27,326	$ 14,645	$ 14,656

[1] Includes after-tax investment and derivative gains/losses and a one-time income tax net benefit attributable to the enactment of the Tax Cuts and Jobs Act of 2017 as follows: 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter Investment and derivative gains/losses - 2017 $ 504 $ 143 $ 623 $ 107 Investment and derivative gains/losses - 2016 1,852 394 2,347 1,904 Income tax net benefit - Tax Cuts and Jobs Act of 2017 - - - 28,200