Short-Term Borrowings (Details) (USD $)	12 Months Ended
	Mar. 31, 2012	May 23, 2012
Unsecured revolving credit agreement
Short-term Debt
Amount available under credit facility	$ 500,000,000
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest rate spread over LIBOR (in bps)	262.5
Effective interest rate for revolving credit agreement (as a percent)	2.90%
Outstanding borrowings under credit facility	250,000,000
Maximum net debt to EBITDA ratio	2.5
Minimum EBITDA to interest ratio	4.0
Net debt to EBITDA ratio	1.1
EBITDA to interest expense ratio	13.8
Short-term borrowings used for repurchasing		250,000,000
Additional borrowings under revolving credit facility		250,000,000
Line of Credit
Short-term Debt
Amount available under credit facility	$ 15,000,000