Income taxes - Reconciliation of Income Tax Expenses Computed at Federal Statutory Rate with Income Tax Expense (Additional Information) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income tax rate	35.00%	35.00%	35.00%
Income Tax Contingency [Line Items]
Total income tax benefit resulting from the 2017 Act	$ 7.8
State
Income Tax Contingency [Line Items]
Total income tax benefit resulting from the 2017 Act	$ 0.6