Stockholder's equity (Details) (USD $) Share data in Millions	12 Months Ended
	Mar. 31, 2011	Mar. 31, 2010	Mar. 31, 2009
Notes to Financial Statements [Abstract]
Number of common stock shares authorized to be repurchased (in shares)	10.0
Repurchased shares under authorization (in shares)	7.5
Amount of repurchased shares under authorization	$ 234,700,000
Number of common stock shares, concurrent with junior subordinated convertible debenture, authorized to be repurchased (in shares)	21.5
Amount of common stock shares, concurrent with junior subordinated convertible debenture, authorized to be repurchased	638,600,000
Purchased amount of its own common stock	$ 0	$ 0	$ 123,929,000
Number of shares remained as treasury (in shares)	29.2