Summary of significant accounting policies (General and administrative costs) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Taxes, Other	$ 1.3	$ 0.9	$ 4.3	$ 3.1

Taxes, excluding payroll, income and excise taxes, if not included elsewhere, that could include production, real and personal property, and other selling and distribution-related taxes.

No definition available.

Name:	us-gaap_TaxesOther
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration