Income taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2020	Dec. 31, 2017
Operating loss carry-forward
Deferred tax assets, valuation allowance		$ 255.9
AMT credit carryforward			$ 4.1
AMT credit carryforward received	$ 2.0
Internal Revenue Service (IRS) | Federal
Operating loss carry-forward
Operating loss carryforwards		2,000.0
State of Oklahoma | State
Operating loss carry-forward
Operating loss carryforwards		34.7
Texas | State
Operating loss carry-forward
Deferred tax liability		4.9
Accounts receivable
Operating loss carry-forward
AMT credit carryforward		$ 2.1