Credit facility (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2012	Aug. 12, 2011
Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	August 12, 2011
Line of Credit Facility, Maximum Borrowing Capacity		$ 750
Line of Credit Facility, Expiration Date	August 12, 2016
Line of Credit Facility Increase Option		250
Line of Credit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Period For Leverage Ratio To Determine Interest Rate Spread	preceding four fiscal quarter period
Debt Instrument Interest Rate Spread On Overdue Principal In Event Of Default		2.00%
Debt Instrument Interest Rate Spread On Overdue Amounts In Event Of Default		2.00%
Line of Credit Facility Foreign Currency Sublimit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		100
Line of Credit Facility Swingline Loan Sublimit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		15
Line of Credit Facility Letter of Credit Sublimit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 25
Debt Instrument Interest Rate Option1 [Member] | Line of Credit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	the highest of JPMorgan Chase Bank, N.A.'s prime rate, the federal funds rate plus a margin equal to 0.50% and the adjusted LIBOR rate for a 1-month interest period plus a margin equal to 1.00%
Debt Instrument Additional Basis On Variable Rate Base		0.50%
Debt Instrument Interest Rate Option1 [Member] | Minimum [Member] | Line of Credit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		0.50%
Debt Instrument Interest Rate Option1 [Member] | Maximum [Member] | Line of Credit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.50%
Debt Instrument Interest Rate Option 2 [Member] | Line of Credit [Member] | Credit Agreement Maturity August 12, 2016 [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	adjusted LIBOR rate (based on one, two, three, or six-month interest periods)
Debt Instrument Additional Basis On Variable Rate Base		1.00%
Debt Instrument Interest Rate Option 2 [Member] | Minimum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		1.50%
Debt Instrument Interest Rate Option 2 [Member] | Maximum [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate		2.50%