Income Taxes (Effective Income Tax Rate Reconciliation) (Details)	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Income Tax Expense (Benefit) [Abstract]
Taxes at U.S. federal statutory rate	35.00%	35.00%	35.00%
Increase in income taxes resulting from:
State income taxes, net of federal benefit	2.90%	2.50%	3.40%
Stock-based compensation expense	0.20%	0.20%	0.10%
State tax effect of corporate reorganization	0.00%	(0.70%)	0.00%
Domestic production activities deduction	(0.90%)	(0.40%)	(0.70%)
Other, net	0.10%	(0.30%)	0.70%
Effective income tax rate	37.30%	36.30%	38.50%