Quarterly data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information [Line Items]
Revenues	$ 65,368		$ 64,972		$ 63,598		$ 60,678		$ 63,714		$ 63,450		$ 62,200		$ 58,473		$ 254,616	$ 247,837	$ 239,933
Net earnings (loss) attributable to Berkshire shareholders	29,159	[1]	16,524	[1]	14,073	[1]	21,661	[1]	(25,392)	[1]	18,540	[1]	12,011	[1]	(1,138)	[1]	$ 81,417	$ 4,021	$ 44,940
After-tax investment and derivative gains/losses	$ 24,739		$ 8,666		$ 7,934		$ 16,106		$ (28,089)		$ 11,660		$ 5,118		$ (6,426)
Equivalent Class A [Member]
Quarterly Financial Information [Line Items]
Net earnings (loss) attributable to Berkshire shareholders per equivalent Class A common share	$ 17,909		$ 10,119		$ 8,608		$ 13,209		$ (15,467)		$ 11,280		$ 7,301		$ (692)		$ 49,828	$ 2,446	$ 27,326

[1] * Includes after-tax investment and derivative gains/losses as follows:       1^st Quarter     2^nd Quarter     3^rd Quarter     4^th Quarter   2019   $ 16,106     $ 7,934     $ 8,666     $ 24,739   2018     (6,426 )     5,118       11,660       (28,089 )