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Schedule of Effective Tax Rates Vary from Federal Statutory Rates (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2016
Mar. 31, 2015
Mar. 31, 2014
Income Tax Disclosure [Abstract]                      
Earnings Before Income Taxes $ 52,322 $ 68,205 $ 42,840 $ 55,885 $ 44,698 $ 77,866 $ 74,577 $ 55,786 $ 219,252 $ 252,927 $ 181,804
Income Taxes at Statutory Rate                 76,738 88,524 63,631
Increases (Decreases) in Tax Resulting from - State Income Taxes, net                 1,166 4,582 173
Increases (Decreases) in Tax Resulting from - Statutory Depletion in Excess of Cost                 (5,672) (4,367) (3,512)
Increases (Decreases) in Tax Resulting from - Domestic Production Activities Deduction                 (6,302) (6,853) (3,453)
Increases (Decreases) in Tax Resulting from - Meals and Entertainment Disallowance                 629 647 503
Increases (Decreases) in Tax Resulting from - Penalties on Uncertain Tax Positions                     213
Increases (Decreases) in Tax Resulting from - IRS Settlement                   (16,559)  
Increases (Decreases) in Tax Resulting from - Other                 101 100 6
Provision for Income Taxes                 $ 66,660 $ 66,074 $ 57,561
Effective Tax Rate                 30.00% 26.00% 32.00%