Condensed Consolidating Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016
Current Assets -
Cash and Cash Equivalents	$ 12,233	$ 6,561	$ 9,180	$ 5,391
Accounts and Notes Receivable	175,002	136,313
Inventories	244,886	252,846
Prepaid and Other Current Assets	8,181	4,904
Total Current Assets	440,302	400,624
Property, Plant and Equipment -	2,454,800	2,439,438
Less: Accumulated Depreciation	(919,732)	(892,601)
Property, Plant and Equipment, net	1,535,068	1,546,837
Notes Receivable	653	815
Investment in Joint Venture	53,750	48,620
Goodwill and Intangible Assets, net	234,707	235,505
Other Assets	15,110	14,723
Total Assets	2,279,590	2,247,124
Current Liabilities -
Accounts Payable	78,763	92,193
Accrued Liabilities	53,288	55,379
Income Tax Payable	26,462	733
Current Portion of Long-term Debt	81,214	81,214
Total Current Liabilities	239,727	229,519
Long-term Debt	580,421	605,253
Other Long-term Liabilities	42,026	42,878
Deferred Income Taxes	162,329	166,024
Total Liabilities	1,024,503	1,043,674
Total Stockholders’ Equity	1,255,087	1,203,450
Liabilities and Stockholders' Equity, Total	2,279,590	2,247,124
Eliminations [Member]
Current Assets -
Income Tax Receivable	(10,252)	(33,196)
Total Current Assets	(10,252)	(33,196)
Investments in Subsidiaries and Receivables from Affiliates	(8,539,710)	(8,378,598)
Total Assets	(8,549,962)	(8,411,794)
Current Liabilities -
Income Tax Payable	(10,252)	(33,196)
Total Current Liabilities	(10,252)	(33,196)
Payables to Affiliates	(6,170,959)	(6,078,019)
Total Liabilities	(6,181,211)	(6,111,215)
Total Stockholders’ Equity	(2,368,751)	(2,300,579)
Liabilities and Stockholders' Equity, Total	(8,549,962)	(8,411,794)
Parent [Member]
Current Assets -
Cash and Cash Equivalents	10,682	5,184	4,208	3,507
Accounts and Notes Receivable	399	422
Income Tax Receivable	10,252	33,196
Prepaid and Other Current Assets	689	484
Total Current Assets	22,022	39,286
Property, Plant and Equipment -	2,925	2,914
Less: Accumulated Depreciation	(968)	(937)
Property, Plant and Equipment, net	1,957	1,977
Investment in Joint Venture	56	51
Investments in Subsidiaries and Receivables from Affiliates	5,230,160	5,126,289
Other Assets	5,605	5,687
Total Assets	5,259,800	5,173,290
Current Liabilities -
Accounts Payable	6,139	6,687
Accrued Liabilities	21,225	21,043
Income Tax Payable		733
Current Portion of Long-term Debt	81,214	81,214
Total Current Liabilities	108,578	109,677
Long-term Debt	580,421	605,253
Other Long-term Liabilities	173	189
Payables to Affiliates	3,309,550	3,252,309
Deferred Income Taxes	5,991	2,412
Total Liabilities	4,004,713	3,969,840
Total Stockholders’ Equity	1,255,087	1,203,450
Liabilities and Stockholders' Equity, Total	5,259,800	5,173,290
Guarantor Subsidiaries [Member]
Current Assets -
Cash and Cash Equivalents	1,551	1,377	$ 4,972	$ 1,884
Accounts and Notes Receivable	174,603	135,891
Inventories	244,886	252,846
Prepaid and Other Current Assets	7,492	4,420
Total Current Assets	428,532	394,534
Property, Plant and Equipment -	2,451,875	2,436,524
Less: Accumulated Depreciation	(918,764)	(891,664)
Property, Plant and Equipment, net	1,533,111	1,544,860
Notes Receivable	653	815
Investment in Joint Venture	53,694	48,569
Investments in Subsidiaries and Receivables from Affiliates	3,309,550	3,252,309
Goodwill and Intangible Assets, net	234,707	235,505
Other Assets	9,505	9,036
Total Assets	5,569,752	5,485,628
Current Liabilities -
Accounts Payable	72,624	85,506
Accrued Liabilities	32,063	34,336
Income Tax Payable	36,714	33,196
Total Current Liabilities	141,401	153,038
Other Long-term Liabilities	41,853	42,689
Payables to Affiliates	2,861,409	2,825,710
Deferred Income Taxes	156,338	163,612
Total Liabilities	3,201,001	3,185,049
Total Stockholders’ Equity	2,368,751	2,300,579
Liabilities and Stockholders' Equity, Total	$ 5,569,752	$ 5,485,628