Revenue Recognition, Consolidated Statement of Operations (Details) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Mar. 31, 2017
Consolidated Statement of Operations [Abstract]
Net sales	$ 127,939,000	$ 110,261,000	$ 94,695,000	$ 219,607,000	$ 204,956,000
Cost of goods sold	102,228,000	84,234,000	68,843,000	177,544,000	153,077,000
Gross profit	25,711,000	26,027,000	25,852,000	42,063,000	51,879,000
Operating expenses [Abstract]
General and administrative	8,997,000	8,615,000	5,888,000	21,088,000	14,503,000
Sales and marketing	4,537,000	3,457,000	3,394,000	8,929,000	6,851,000
Research and development	1,784,000	1,240,000	1,002,000	3,520,000	2,242,000
Total operating expenses	15,318,000	13,312,000	10,284,000	33,537,000	23,596,000
Operating income	10,393,000	12,715,000	15,568,000	8,526,000	28,283,000
Interest expense, net	5,699,000	3,522,000	3,314,000	10,774,000	6,836,000
Income before income tax expense (benefit)	4,694,000	9,193,000	12,254,000	(2,248,000)	21,447,000
Income tax expense (benefit)	1,181,000	3,598,000	4,434,000	(266,000)	8,032,000
Net income (loss)	$ 3,513,000	$ 5,595,000	$ 7,820,000	$ (1,982,000)	$ 13,415,000
Basic net income (loss) per share (in dollars per share)	$ 0.19	$ 0.30	$ 0.42	$ (0.10)	$ 0.72
Diluted net income (loss) per share (in dollars per share)	$ 0.18	$ 0.29	$ 0.40	$ (0.10)	$ 0.69
As Revised [Member] | ASC 606 [Member]
Consolidated Statement of Operations [Abstract]
Net sales		$ 110,853,000	$ 94,239,000		$ 205,092,000
Cost of goods sold		84,612,000	69,224,000		153,836,000
Gross profit		26,241,000	25,015,000		51,256,000
Operating expenses [Abstract]
General and administrative		8,615,000	5,888,000		14,503,000
Sales and marketing		3,457,000	3,394,000		6,851,000
Research and development		1,240,000	1,002,000		2,242,000
Total operating expenses		13,312,000	10,284,000		23,596,000
Operating income		12,929,000	14,731,000		27,660,000
Interest expense, net		3,522,000	3,314,000		6,836,000
Income before income tax expense (benefit)		9,407,000	11,417,000		20,824,000
Income tax expense (benefit)		3,684,000	4,122,000		7,806,000
Net income (loss)		$ 5,723,000	$ 7,295,000		$ 13,018,000
Basic net income (loss) per share (in dollars per share)		$ 0.31	$ 0.39		$ 0.70
Diluted net income (loss) per share (in dollars per share)		$ 0.30	$ 0.38		$ 0.67
Adoption of ASC 606 [Member] | ASC 606 [Member]
Consolidated Statement of Operations [Abstract]
Net sales		$ (592,000)	$ 456,000		$ (136,000)	$ 557,000	$ (824,000)
Cost of goods sold		(378,000)	(381,000)		(759,000)	$ 66,000	$ (758,000)
Gross profit		(214,000)	837,000		623,000
Operating expenses [Abstract]
General and administrative		0	0		0
Sales and marketing		0	0		0
Research and development		0	0		0
Total operating expenses		0	0		0
Operating income		(214,000)	837,000		623,000
Interest expense, net		0	0		0
Income before income tax expense (benefit)		(214,000)	837,000		623,000
Income tax expense (benefit)		(86,000)	312,000		226,000
Net income (loss)		$ (128,000)	$ 525,000		$ 397,000
Basic net income (loss) per share (in dollars per share)		$ (0.01)	$ 0.03		$ 0.02
Diluted net income (loss) per share (in dollars per share)		$ (0.01)	$ 0.03		$ 0.02