Tax credit carryforward (Details) $ in Millions	12 Months Ended
	Mar. 31, 2018 USD ($)
Tax Credit Carryforward [Line Items]
Refundable foreign tax credits	$ 49.2
Foreign Tax Authority | Foreign Withholding Tax Credit
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	20.0
Tax credit carryforward, valuation allowance	$ 20.0
Foreign Tax Authority | Foreign Withholding Tax Credit | Minimum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2022
Foreign Tax Authority | Foreign Withholding Tax Credit | Maximum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2024
State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	$ 96.3
Tax credit carryforward, valuation allowance	$ 96.3
State and Local Jurisdiction | Minimum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2018
State and Local Jurisdiction | Maximum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2037