Acquisitions (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020 USD ($) Store	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) Store Acquisition	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Aug. 01, 2019 Store	May 01, 2019 Store	Feb. 01, 2019 Store	Dec. 01, 2018 Store	Jun. 01, 2018 Store	Mar. 31, 2018 Store	Feb. 01, 2018 Store
Acquisition [Abstract]
Number of acquisitions completed | Acquisition									0
Number of stores | Store	61								61
Revenue	$ 271,036	$ 408,273	$ 189,963	$ 153,698	$ 208,751	$ 274,824	$ 180,771	$ 103,278	$ 1,022,970
Net income	$ 1,973	$ 14,367	$ 1,085	$ 0	0	$ 0	$ 0	$ 0	17,425
Costs related to acquisition									3,648	$ 1,323
Payment of contingent consideration									$ 1,456	0	$ 0
Slalom Shop [Member]
Acquisition [Abstract]
Number of stores | Store															2
Ocean Blue [Member]
Acquisition [Abstract]
Number of stores | Store														3
Ray Clepper [Member]
Acquisition [Abstract]
Number of stores | Store														1
Caribee [Member]
Acquisition [Abstract]
Number of stores | Store													1
Central Marine [Member]
Acquisition [Abstract]
Number of stores | Store												3
Slalom Shop, Ocean Blue, Ray Clepper, Caribee and Central Marine [Member]
Acquisition [Abstract]
Total purchase price					$ 48,641					48,641
Cash paid for acquisition										19,400
Slalom Shop, Ocean Blue, Ray Clepper, Caribee and Central Marine [Member] | Long-term Debt [Member]
Acquisition [Abstract]
Note payable to seller										29,200
Retail Boat Dealer Groups [Member]
Acquisition [Abstract]
Revenue										62,000	68,400
Net income										$ 4,000	6,100
Texas Marine [Member]
Acquisition [Abstract]
Number of stores | Store																		3
Spend-A- Day [Member]
Acquisition [Abstract]
Number of stores | Store																	2
Bosun [Member]
Acquisition [Abstract]
Number of stores | Store																4
Texas Marine, Spend-A-Day Marina and Bosun's [Member]
Acquisition [Abstract]
Total purchase price											31,000
Cash paid for acquisition											13,800
Costs related to acquisition											438
Payment of contingent consideration											2,600
Amount of investment purchase price											2,500
Texas Marine, Spend-A-Day Marina and Bosun's [Member] | Long-term Debt [Member]
Acquisition [Abstract]
Note payable to seller											$ 12,000