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Schedule of Components of Deferred Income Taxes (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Mar. 31, 2016
Income Tax Disclosure [Abstract]    
Items Giving Rise to Deferred Tax Liabilities - Excess Tax Depreciation and Amortization $ (161,204) $ (157,260)
Items Giving Rise to Deferred Tax Liabilities - Depletion (18,638) (21,911)
Items Giving Rise to Deferred Tax Liabilities - Repair Parts (1,747) (2,446)
Items Giving Rise to Deferred Tax Liabilities - State Income Taxes, net (8,883) (12,552)
Items Giving Rise to Deferred Tax Liabilities - Other (8,515) (4,147)
Items Giving Rise to Deferred Tax Liabilities - Total Deferred Tax Liabilities (198,987) (198,316)
Items Giving Rise to Deferred Tax Assets - Accrual Changes 12,992 11,855
Items Giving Rise to Deferred Tax Assets - Bad Debts 3,895 3,604
Items Giving Rise to Deferred Tax Assets - Uniform Capitalization 2,989 2,174
Items Giving Rise to Deferred Tax Assets - Deferred Revenue   700
Items Giving Rise to Deferred Tax Assets - Long-term Incentive Compensation Plan 8,691 11,458
Items Giving Rise to Deferred Tax Assets - Pension and Other 4,396 6,846
Items Giving Rise to Deferred Tax Assets - Total Deferred Tax Assets $ 32,963 $ 36,637