Income tax (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Income Tax Disclosure [Abstract]
Effective Income Tax Rate	38.10%	12.50%
Unrecognized Tax Benefits	$ 179.8
Unrecognized Tax Benefits, Period Increase (Decrease)	$ 109.3