Quarterly data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information [Line Items]
Revenues	$ 64,381		$ 63,024		$ 56,840		$ 61,265		$ 65,368		$ 64,972		$ 63,598		$ 60,678		$ 245,510	$ 254,616	$ 247,837
Net earnings (loss) attributable to Berkshire shareholders	35,835	[1]	30,137	[1]	26,295	[1]	(49,746)	[1]	29,159	[1]	16,524	[1]	14,073	[1]	21,661	[1]	$ 42,521	$ 81,417	$ 4,021
After-tax investment and derivative gains/losses	$ 30,826		$ 24,737		$ 31,645		$ (55,617)		$ 24,739		$ 8,666		$ 7,934		$ 16,106
Equivalent Class A [Member]
Quarterly Financial Information [Line Items]
Net earnings (loss) attributable to Berkshire shareholders per equivalent Class A common share	$ 23,015		$ 18,994		$ 16,314		$ (30,653)		$ 17,909		$ 10,119		$ 8,608		$ 13,209		$ 26,668	$ 49,828	$ 2,446

[1] * Includes after-tax investment and derivative gains/losses as follows:       1^st Quarter     2^nd Quarter     3^rd Quarter     4^th Quarter   2020   $ (55,617 )   $ 31,645     $ 24,737     $ 30,826   2019     16,106       7,934       8,666       24,739