Revenue Recognition, Consolidated Balance Sheet (Details) - USD ($)		Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Current assets [Abstract]
Cash and cash equivalents		$ 8,591,000	$ 13,049,000
Short-term investments		2,868,000	2,828,000
Accounts receivable - net		54,761,000	63,174,000
Inventory net		205,075,000	161,210,000
Inventory unreturned		10,746,000	7,508,000
Contract assets		26,965,000	23,206,000
Income tax receivable		12,887,000	7,972,000
Prepaid expenses and other current assets		7,946,000	8,608,000
Total current assets		329,839,000	287,555,000
Plant and equipment - net		32,349,000	28,322,000
Long-term core inventory - net			0
Long-term core inventory deposits			0
Long-term deferred income taxes		7,607,000	6,698,000
Long-term contract assets		222,999,000	222,731,000
Goodwill		3,402,000	2,551,000	$ 2,551,000	$ 2,551,000
Intangible assets - net		8,762,000	3,766,000
Other assets		891,000	804,000
TOTAL ASSETS		605,849,000	552,427,000
Current liabilities [Abstract]
Accounts payable		101,730,000	73,273,000
Accrued liabilities		13,382,000	12,048,000
Customer finished goods returns accrual		19,236,000	17,805,000
Accrued core payment			0
Contract liabilities		29,239,000	32,603,000
Revolving loan		78,406,000	54,000,000
Other current liabilities		5,019,000	4,471,000
Current portion of term loan		3,685,000	3,068,000
Total current liabilities		250,697,000	197,268,000
Term loan, less current portion		25,109,000	13,913,000
Long-term accrued core payment			0
Long-term deferred income taxes		234,000	226,000
Long-term contract liabilities		42,527,000	48,183,000
Other liabilities		7,494,000	5,957,000
Total liabilities		326,061,000	265,547,000
Commitments and contingencies
Shareholders' equity [Abstract]
Preferred stock		0	0
Common stock; par value $.01 per share, 50,000,000 shares authorized; 18,893,102 shares issued and outstanding at March 31, 2018		188,000	189,000
Additional paid-in capital		212,621,000	213,609,000
Retained earnings		74,172,000	78,510,000
Accumulated other comprehensive loss		(7,193,000)	(5,428,000)
Total shareholders' equity		279,788,000	286,880,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY		$ 605,849,000	$ 552,427,000
Balance Sheet Parenthetical Information [Abstract]
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Preferred stock, authorized (in shares)		5,000,000	5,000,000
Preferred stock, issued (in shares)		0	0
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock, authorized (in shares)		50,000,000	50,000,000
Common stock, issued (in shares)		18,812,102	18,893,102
Common stock, outstanding (in shares)		18,812,102	18,893,102
Series A Junior Participating Preferred Stock [Member]
Shareholders' equity [Abstract]
Preferred stock		$ 0	$ 0
Balance Sheet Parenthetical Information [Abstract]
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Preferred stock, authorized (in shares)		20,000	20,000
Preferred stock, issued (in shares)		0	0
As Revised [Member] | ASC 606 [Member]
Current assets [Abstract]
Cash and cash equivalents			$ 13,049,000
Short-term investments			2,828,000
Accounts receivable - net			15,738,000
Inventory net			76,275,000
Inventory unreturned			7,508,000
Contract assets			0
Income tax receivable			7,972,000
Prepaid expenses and other current assets			15,104,000
Total current assets			138,474,000
Plant and equipment - net			28,322,000
Long-term core inventory - net			298,294,000
Long-term core inventory deposits			5,569,000
Long-term deferred income taxes			6,937,000
Long-term contract assets			0
Goodwill			2,551,000
Intangible assets - net			3,766,000
Other assets			21,995,000
TOTAL ASSETS			505,908,000
Current liabilities [Abstract]
Accounts payable			73,273,000
Accrued liabilities			12,048,000
Customer finished goods returns accrual			17,805,000
Accrued core payment			16,536,000
Contract liabilities			0
Revolving loan			54,000,000
Other current liabilities			4,471,000
Current portion of term loan			3,068,000
Total current liabilities			181,201,000
Term loan, less current portion			13,913,000
Long-term accrued core payment			18,473,000
Long-term deferred income taxes			226,000
Long-term contract liabilities			0
Other liabilities			5,957,000
Total liabilities			219,770,000
Commitments and contingencies
Shareholders' equity [Abstract]
Preferred stock			0
Common stock; par value $.01 per share, 50,000,000 shares authorized; 18,893,102 shares issued and outstanding at March 31, 2018			189,000
Additional paid-in capital			213,609,000
Retained earnings			77,768,000
Accumulated other comprehensive loss			(5,428,000)
Total shareholders' equity			286,138,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY			505,908,000
As Revised [Member] | ASC 606 [Member] | Series A Junior Participating Preferred Stock [Member]
Shareholders' equity [Abstract]
Preferred stock			0
Adoption of ASC 606 [Member] | ASC 606 [Member]
Current assets [Abstract]
Cash and cash equivalents			0
Short-term investments			0
Accounts receivable - net			47,436,000
Inventory net			84,935,000
Inventory unreturned			0
Contract assets			23,206,000
Income tax receivable			0
Prepaid expenses and other current assets			(6,496,000)
Total current assets			149,081,000
Plant and equipment - net			0
Long-term core inventory - net			(298,294,000)
Long-term core inventory deposits	[1]		(5,569,000)
Long-term deferred income taxes			(239,000)
Long-term contract assets			222,731,000
Goodwill			0
Intangible assets - net			0
Other assets			(21,191,000)
TOTAL ASSETS			46,519,000
Current liabilities [Abstract]
Accounts payable			0
Accrued liabilities			0
Customer finished goods returns accrual			0
Accrued core payment			(16,536,000)
Contract liabilities			32,603,000
Revolving loan			0
Other current liabilities			0
Current portion of term loan			0
Total current liabilities			16,067,000
Term loan, less current portion			0
Long-term accrued core payment			(18,473,000)
Long-term deferred income taxes			0
Long-term contract liabilities			48,183,000
Other liabilities			0
Total liabilities			45,777,000
Commitments and contingencies
Shareholders' equity [Abstract]
Preferred stock			0
Common stock; par value $.01 per share, 50,000,000 shares authorized; 18,893,102 shares issued and outstanding at March 31, 2018			0
Additional paid-in capital			0
Retained earnings			742,000			$ 345,000
Accumulated other comprehensive loss			0
Total shareholders' equity			742,000
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY			46,519,000
Adoption of ASC 606 [Member] | ASC 606 [Member] | Series A Junior Participating Preferred Stock [Member]
Shareholders' equity [Abstract]
Preferred stock			$ 0

[1] Long-term core inventory deposits of $5,569,000 have been reclassified to long-term contract assets in the consolidated balance sheet at March 31, 2018.