Schedule of Effective Tax Rates Vary from Federal Statutory Rates (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Earnings Before Income Taxes	$ 48,725	$ 51,388	$ 92,319	$ 79,530	$ 54,181	$ 83,689	$ 89,373	$ 67,276	$ 271,962	$ 294,519	$ 219,252
Income Taxes at Statutory Rate									85,804	103,082	76,738
Increases (Decreases) in Tax Resulting from - State Income Taxes, net									6,139	1,668	1,166
Increases (Decreases) in Tax Resulting from - Statutory Depletion in Excess of Cost									(6,341)	(1,216)	(5,672)
Increases (Decreases) in Tax Resulting from - Domestic Production Activities Deduction									(5,995)	(7,615)	(6,302)
Increases (Decreases) in Tax Resulting from - Excess Tax Benefit from Stock Compensation									(4,129)
Increases (Decreases) in Tax Resulting from - Meals and Entertainment Disallowance									567	626	629
Increases (Decreases) in Tax Resulting from - Limitation on Officer's Compensation									246
Increases (Decreases) in Tax Resulting from - Impact of Tax Cuts and Jobs Act of 2017									(61,692)
Increases (Decreases) in Tax Resulting from - Other									731	(245)	101
Provision for Income Taxes									$ 15,330	$ 96,300	$ 66,660
Effective Tax Rate									6.00%	33.00%	30.00%