Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Current Assets -
Cash and Cash Equivalents	$ 9,315	$ 6,561
Restricted Cash	38,753
Accounts and Notes Receivable	141,685	136,313
Inventories	258,159	252,846
Income Tax Receivable	5,750
Prepaid and Other Current Assets	5,073	4,904
Total Current Assets	458,735	400,624
Property, Plant, and Equipment -	2,586,528	2,439,438
Less: Accumulated Depreciation	(991,229)	(892,601)
Property, Plant, and Equipment, net	1,595,299	1,546,837
Notes Receivable	115	815
Investment in Joint Venture	60,558	48,620
Goodwill and Intangible Assets, net	239,342	235,505
Other Assets	13,954	14,723
Total Assets	2,368,003	2,247,124	$ 1,883,635
Current Liabilities -
Accounts Payable	73,459	92,193
Accrued Liabilities	105,870	55,379
Income Tax Payable		733
Current Portion of Long-term Debt		81,214
Total Current Liabilities	179,329	229,519
Long-term Debt	620,922	605,253
Other Long-term Liabilities	31,096	42,878
Deferred Income Taxes	118,966	166,024
Total Liabilities	950,313	1,043,674
Total Stockholders’ Equity	1,417,690	1,203,450	$ 1,040,531	$ 1,010,593
Liabilities and Stockholders' Equity, Total	2,368,003	2,247,124
Eliminations [Member]
Current Assets -
Income Tax Receivable	(103,760)	(33,196)
Total Current Assets	(103,760)	(33,196)
Investments in Subsidiaries and Receivables from Affiliates	(3,481,149)	(8,378,598)
Total Assets	(3,584,909)	(8,411,794)
Current Liabilities -
Income Tax Payable	(103,760)	(33,196)
Total Current Liabilities	(103,760)	(33,196)
Payables to Affiliates	(6,370,576)	(6,078,019)
Total Liabilities	(6,474,336)	(6,111,215)
Total Stockholders’ Equity	2,889,427	(2,300,579)
Liabilities and Stockholders' Equity, Total	(3,584,909)	(8,411,794)
Parent [Member]
Current Assets -
Cash and Cash Equivalents	5,784	5,184
Restricted Cash	38,753
Accounts and Notes Receivable	407	422
Income Tax Receivable	109,510	33,196
Prepaid and Other Current Assets	665	484
Total Current Assets	155,119	39,286
Property, Plant, and Equipment -	3,188	2,914
Less: Accumulated Depreciation	(1,089)	(937)
Property, Plant, and Equipment, net	2,099	1,977
Investment in Joint Venture	70	51
Investments in Subsidiaries and Receivables from Affiliates	2,718,809	5,126,289
Other Assets	5,417	5,687
Total Assets	2,881,514	5,173,290
Current Liabilities -
Accounts Payable	5,591	6,687
Accrued Liabilities	67,387	21,043
Income Tax Payable		733
Current Portion of Long-term Debt		81,214
Total Current Liabilities	72,978	109,677
Long-term Debt	620,922	605,253
Other Long-term Liabilities	124	189
Payables to Affiliates	762,340	3,252,309
Deferred Income Taxes	7,460	2,412
Total Liabilities	1,463,824	3,969,840
Total Stockholders’ Equity	1,417,690	1,203,450
Liabilities and Stockholders' Equity, Total	2,881,514	5,173,290
Guarantor Subsidiaries [Member]
Current Assets -
Cash and Cash Equivalents	3,531	1,377
Accounts and Notes Receivable	141,278	135,891
Inventories	258,159	252,846
Prepaid and Other Current Assets	4,408	4,420
Total Current Assets	407,376	394,534
Property, Plant, and Equipment -	2,583,340	2,436,524
Less: Accumulated Depreciation	(990,140)	(891,664)
Property, Plant, and Equipment, net	1,593,200	1,544,860
Notes Receivable	115	815
Investment in Joint Venture	60,488	48,569
Investments in Subsidiaries and Receivables from Affiliates	762,340	3,252,309
Goodwill and Intangible Assets, net	239,342	235,505
Other Assets	8,537	9,036
Total Assets	3,071,398	5,485,628
Current Liabilities -
Accounts Payable	67,868	85,506
Accrued Liabilities	38,483	34,336
Income Tax Payable	103,760	33,196
Total Current Liabilities	210,111	153,038
Other Long-term Liabilities	30,972	42,689
Payables to Affiliates	5,608,236	2,825,710
Deferred Income Taxes	111,506	163,612
Total Liabilities	5,960,825	3,185,049
Total Stockholders’ Equity	(2,889,427)	2,300,579
Liabilities and Stockholders' Equity, Total	$ 3,071,398	$ 5,485,628