Inventories (Q3) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Component of Inventory [Abstract]
Inventories	$ 171,300	$ 277,338
New Vessels [Member]
Component of Inventory [Abstract]
Inventories	135,229	234,312
Pre-owned Vessels [Member]
Component of Inventory [Abstract]
Inventories	26,956	33,729
Work in Process, Parts and Accessories [Member]
Component of Inventory [Abstract]
Inventories	$ 9,115	$ 9,297

Amount after valuation and LIFO reserves of inventory expected to be sold, or consumed within one year or operating cycle, if longer.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121566466&loc=d3e6676-107765

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.6(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Name:	us-gaap_InventoryNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant