Goodwill and intangible assets - Information Regarding Company's Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2020
Schedule Of Intangible Assets And Goodwill [Line Items]
Total finite-lived intangibles, Gross carrying amount	$ 85,040	$ 72,700	$ 85,040
Total finite-lived intangibles, Accumulated amortization	(50,492)	(42,887)	(46,430)
Total	34,548	29,813	38,610
Goodwill, Gross carrying amount	171,620	157,264	171,321
Goodwill, Accumulated amortization	0	0	0
Goodwill, Net carrying amount	171,620	157,264	171,321
Total goodwill and other intangibles, Gross carrying amount	320,460	293,764	320,161
Total goodwill and other intangibles, Accumulated amortization	(50,492)	(42,887)	(46,430)
Total goodwill and other intangibles, Net carrying amount	269,968	250,877	273,731
Trademarks
Schedule Of Intangible Assets And Goodwill [Line Items]
Indefinite lived intangibles, Net carrying amount	$ 63,800	$ 63,800	$ 63,800
Customer relationships – retailers
Schedule Of Intangible Assets And Goodwill [Line Items]
Estimated useful life	10 years	10 years	10 years
Total finite-lived intangibles, Gross carrying amount	$ 77,600	$ 68,800	$ 77,600
Total finite-lived intangibles, Accumulated amortization	(46,380)	(38,987)	(42,500)
Total	$ 31,220	$ 29,813	$ 35,100
Customer relationships – e-commerce
Schedule Of Intangible Assets And Goodwill [Line Items]
Estimated useful life	3 years	3 years	3 years
Total finite-lived intangibles, Gross carrying amount	$ 3,940	$ 3,900	$ 3,940
Total finite-lived intangibles, Accumulated amortization	(3,908)	(3,900)	(3,901)
Total	$ 32	$ 0	$ 39
Trademarks
Schedule Of Intangible Assets And Goodwill [Line Items]
Estimated useful life	10 years		10 years
Total finite-lived intangibles, Gross carrying amount	$ 3,500		$ 3,500
Total finite-lived intangibles, Accumulated amortization	(204)		(29)
Total	$ 3,296		$ 3,471