Income taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Jun. 30, 2013	Mar. 31, 2012
Income Tax Disclosure [Abstract]
Income tax expense for unrecognized tax positions		$ 2.2
Reduction in deferred income tax expense due to deferred income tax rate reduction		2.5
Unrecognized tax benefits to be settled within 12 months	$ 14.9
Anticipated settlement of federal and state audits	12 months