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Deferred compensation plans - SAR Plan A, Weighted-Average Assumptions (Detail)	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	43.11%	41.78%	40.51%
Expected dividends yield	2.12%	3.31%	1.73%
Expected lives (in years)	7 years	6 years	6 years
Risk-free interest rate	0.45%	0.63%	0.76%

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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