Business Combinations and Dispositions- Schedule of Calculation of Consideration Transferred (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jan. 01, 2021	Sep. 30, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Common stock, shares outstanding		196,288	116,829
Common stock, shares issued		196,288	116,829
BMC Stock Holdings, Inc.
Business Acquisition [Line Items]
Common stock, shares outstanding	67,568
Exchange ratio for common shares outstanding per Merger Agreement	1.3125%
Common stock, shares issued	88,683
Number of BMC stock awards that vested as a result of the BMC Merger	688
Exchange ratio for stock awards expected to vest per Merger Agreement	1.3125%
Shares of BFS common stock issued for BMC vested restricted stock awards	903
Fair value of BFS common stock issued for BMC outstanding common stock and vested equity awards	$ 3,655,988
Fair value of modified BMC fully vested, unexercised options	2,374
Total consideration transferred	$ 3,658,362
BMC Stock Holdings, Inc. | BMC
Business Acquisition [Line Items]
Common stock price per share	$ 40.81