Impact on Previously Issued Financial Statements for the Correction of an Error, Revised Consolidated Statement of Operations (Details) - USD ($)	3 Months Ended					6 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
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
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member]
Quantifying Prior Year Misstatements Corrected in Current Year Financial Statements [Abstract]
Cumulative understatement of net income		$ 2,938,000
Consolidated Statement of Operations [Abstract]
Net sales		91,668,000	$ 101,713,000	$ 110,853,000	$ 94,239,000		$ 205,092,000	$ 306,805,000	$ 426,991,000	$ 422,882,000	$ 369,670,000
Cost of goods sold		75,316,000	75,833,000	84,612,000	69,224,000		153,836,000	229,669,000	320,449,000	305,926,000	268,448,000
Gross profit		16,352,000	25,880,000	26,241,000	25,015,000		51,256,000	77,136,000	106,542,000	116,956,000	101,222,000
Operating expenses [Abstract]
General and administrative		12,091,000	11,915,000	8,615,000	5,888,000		14,503,000	26,418,000	35,477,000	31,125,000	49,963,000
Sales and marketing		4,392,000	4,048,000	3,457,000	3,394,000		6,851,000	10,899,000	15,030,000	12,126,000	9,965,000
Research and development		1,736,000	1,678,000	1,240,000	1,002,000		2,242,000	3,920,000	5,692,000	3,824,000	3,008,000
Total operating expenses		18,219,000	17,641,000	13,312,000	10,284,000		23,596,000	41,237,000	56,199,000	47,075,000	62,936,000
Operating income		(1,867,000)	8,239,000	12,929,000	14,731,000		27,660,000	35,899,000	50,343,000	69,881,000	38,286,000
Interest expense, net		5,075,000	3,953,000	3,522,000	3,314,000		6,836,000	10,789,000	15,445,000	13,094,000	16,244,000
Income before income tax expense (benefit)		(6,942,000)	4,286,000	9,407,000	11,417,000		20,824,000	25,110,000	34,898,000	56,787,000	22,042,000
Income tax expense (benefit)		(1,447,000)	6,936,000	3,684,000	4,122,000		7,806,000	14,742,000	16,072,000	18,011,000	11,773,000
Net income (loss)		$ (5,495,000)	$ (2,650,000)	$ 5,723,000	$ 7,295,000		$ 13,018,000	$ 10,368,000	$ 18,826,000	$ 38,776,000	$ 10,269,000
Basic net income (loss) per share (in dollars per share)		$ (0.29)	$ (0.14)	$ 0.31	$ 0.39		$ 0.70	$ 0.55	$ 1.00	$ 2.08	$ 0.56
Diluted net income (loss) per share (in dollars per share)		$ (0.29)	$ (0.14)	$ 0.30	$ 0.38		$ 0.67	$ 0.53	$ 0.96	$ 2.00	$ 0.54
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member] | As Previously Reported [Member]
Consolidated Statement of Operations [Abstract]
Net sales		$ 92,565,000	$ 100,127,000	$ 111,774,000	$ 95,063,000		$ 206,837,000	$ 306,964,000	$ 428,072,000	$ 421,253,000	$ 368,970,000
Cost of goods sold		75,314,000	77,583,000	84,612,000	69,224,000		153,836,000	231,419,000	322,199,000	306,207,000	268,046,000
Gross profit		17,251,000	22,544,000	27,162,000	25,839,000		53,001,000	75,545,000	105,873,000	115,046,000	100,924,000
Operating expenses [Abstract]
General and administrative		12,340,000	11,915,000	8,615,000	6,187,000		14,802,000	26,717,000	35,527,000	31,124,000	49,665,000
Sales and marketing		4,392,000	4,048,000	3,457,000	3,394,000		6,851,000	10,899,000	15,030,000	12,126,000	9,965,000
Research and development		1,736,000	1,678,000	1,240,000	1,002,000		2,242,000	3,920,000	5,692,000	3,824,000	3,008,000
Total operating expenses		18,468,000	17,641,000	13,312,000	10,583,000		23,895,000	41,536,000	56,249,000	47,074,000	62,638,000
Operating income		(1,217,000)	4,903,000	13,850,000	15,256,000		29,106,000	34,009,000	49,624,000	67,972,000	38,286,000
Interest expense, net		5,075,000	3,953,000	3,522,000	3,314,000		6,836,000	10,789,000	15,445,000	13,094,000	16,244,000
Income before income tax expense (benefit)		(6,292,000)	950,000	10,328,000	11,942,000		22,270,000	23,220,000	34,179,000	54,878,000	22,042,000
Income tax expense (benefit)		(1,278,000)	7,756,000	4,027,000	4,316,000		8,343,000	16,099,000	17,863,000	17,305,000	11,479,000
Net income (loss)		$ (5,014,000)	$ (6,806,000)	$ 6,301,000	$ 7,626,000		$ 13,927,000	$ 7,121,000	$ 16,316,000	$ 37,573,000	$ 10,563,000
Basic net income (loss) per share (in dollars per share)		$ (0.27)	$ (0.36)	$ 0.34	$ 0.41		$ 0.75	$ 0.38	$ 0.87	$ 2.02	$ 0.58
Diluted net income (loss) per share (in dollars per share)		$ (0.27)	$ (0.36)	$ 0.33	$ 0.39		$ 0.72	$ 0.37	$ 0.84	$ 1.93	$ 0.55
Historical Misapplication of GAAP Related to Timing of Recognizing Certain Expenses Incurred in Connection with New Business [Member] | Adjustment [Member]
Consolidated Statement of Operations [Abstract]
Net sales		$ (897,000)	$ 1,586,000	$ (921,000)	$ (824,000)		$ (1,745,000)	$ (159,000)	$ (1,081,000)	$ 1,629,000	$ 700,000
Cost of goods sold		2,000	(1,750,000)	0	0		0	(1,750,000)	(1,750,000)	(281,000)	402,000
Gross profit		(899,000)	3,336,000	(921,000)	(824,000)		(1,745,000)	1,591,000	669,000	1,910,000	298,000
Operating expenses [Abstract]
General and administrative		(249,000)	0	0	(299,000)		(299,000)	(299,000)	(50,000)	1,000	298,000
Sales and marketing		0	0	0	0		0	0	0	0	0
Research and development		0	0	0	0		0	0	0	0	0
Total operating expenses		(249,000)	0	0	(299,000)		(299,000)	(299,000)	(50,000)	1,000	298,000
Operating income		(650,000)	3,336,000	(921,000)	(525,000)		(1,446,000)	1,890,000	719,000	1,909,000	0
Interest expense, net		0	0	0	0		0	0	0	0	0
Income before income tax expense (benefit)		(650,000)	3,336,000	(921,000)	(525,000)		(1,446,000)	1,890,000	719,000	1,909,000	0
Income tax expense (benefit)		(169,000)	(820,000)	(343,000)	(194,000)		(537,000)	(1,357,000)	(1,791,000)	706,000	294,000
Net income (loss)		$ (481,000)	$ 4,156,000	$ (578,000)	$ (331,000)		$ (909,000)	$ 3,247,000	$ 2,510,000	$ 1,203,000	$ (294,000)
Basic net income (loss) per share (in dollars per share)		$ (0.03)	$ 0.22	$ (0.03)	$ (0.02)		$ (0.05)	$ 0.17	$ 0.13	$ 0.06	$ (0.02)
Diluted net income (loss) per share (in dollars per share)		$ (0.03)	$ 0.22	$ (0.03)	$ (0.02)		$ (0.05)	$ 0.17	$ 0.13	$ 0.06	$ (0.02)