Income tax (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended	6 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Effective Income Tax Rate		8.00%	11.50%
Release of prior year tax provisions	$ 6.8	$ 7.4
Release of prior year tax provisions basic earnings per share impact	$ 0.03	$ 0.04
Release of prior year tax provisions diluted earnings per share impact	$ 0.03	$ 0.03
Unrecognized Tax Benefits	212.1	212.1
Unrecognized Tax Benefits, Period Increase (Decrease)		32.1
unrecognized tax benefit presented as a reduction to deferred tax assets for net operating loss carryforward and other tax credit carryforwards	$ 72.7	$ 72.7