Stockholders' and Members' Equity, Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended																12 Months Ended
		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2020	Feb. 10, 2020
Numerator [Abstract]
Net income attributable to OneWater Inc		$ 1,973		$ 14,367		$ 1,085		$ 0		$ 0		$ 0		$ 0		$ 0		$ 17,425
Denominator [Abstract]
Weighted-average number of unrestricted outstanding common shares used to calculate basic net income per share (in shares)																		6,243,000
Effect of dilutive securities:
Restricted stock units (in shares)																		44,000
Diluted weighted-average shares of Class A common stock outstanding used to calculate diluted net income per share (in shares)																		6,287,000
Earnings per share of Class A common stock - basic (in dollars per share)		$ 0.30	[1]	$ 2.36	[1]	$ 0.18	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 2.79
Earnings per share of Class A common stock - diluted (in dollars per share)		$ 0.30	[1]	$ 2.36	[1]	$ 0.18	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 0	[1]	$ 2.77
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)																		8,544,000
Restricted Stock Units [Member]
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)																		220,000
Common Class A [Member]
Per Share Data [Abstract]
Common stock, shares outstanding (in shares)		10,391,661								0								10,391,661	0
Denominator [Abstract]
Weighted-average number of unrestricted outstanding common shares used to calculate basic net income per share (in shares)	[2]																	6,243,000
Effect of dilutive securities:
Diluted weighted-average shares of Class A common stock outstanding used to calculate diluted net income per share (in shares)	[2]																	6,287,000
Earnings per share of Class A common stock - basic (in dollars per share)	[2]																	$ 2.79
Earnings per share of Class A common stock - diluted (in dollars per share)	[2]																	$ 2.77
Common Class B [Member]
Per Share Data [Abstract]
Common stock, shares outstanding (in shares)		4,583,637								0								4,583,637	0
Antidilutive Securities [Abstract]
Antidilutive securities excluded from computation of diluted weighted average shares (in shares)																		8,324,000

[1] Represents earnings per share of Class A common stock and weighted-average shares of Class A common stock outstanding for the period from February 11, 2020 through September 30, 2020, the period following the Organizational Transactions and Offering. See Note 1.

[2] Represents earnings per share of Class A common stock and weighted-average shares of Class A common stock outstanding for the period from February 11, 2020 through September 30, 2020, the period following the Organizational Transactions (as defined below) and OneWater Marine Inc.'s initial public offering. See Note 1.