Basis of Financial Statement Presentation - ASC 606 Adjustment Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales	$ 327,616	$ 273,946
Operating costs and expenses:
Cost of sales	232,795	199,829
Research and development	10,688	9,247
Income tax expense	4,600	1,140
Net income (loss)	$ 35,220	$ 220,439
Net income (loss) per basic share (in usd per share)	$ 0.61	$ 4.65
As Previously Reported
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 274,000
Operating costs and expenses:
Cost of sales		199,563
Research and development		9,390
Income tax expense		1,200
Net income (loss)		$ 220,606
Net income (loss) per basic share (in usd per share)		$ 4.66
Adjustments | Adjustments
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ (54)
Operating costs and expenses:
Cost of sales		266
Research and development		(143)
Income tax expense		0
Net income (loss)		$ (167)
Net income (loss) per basic share (in usd per share)		$ (0.01)