Income taxes (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 11,200,000	$ 2,100,000	$ (3,600,000)	$ 19,300,000
Pre-tax income (loss)	$ 4,000,000	$ 10,300,000	$ (132,100,000)	$ 131,600,000
Effective tax rate (as a percent)				14.50%
Tax Cuts and Jobs Act of 2017, BEAT tax rate, 2019-2025 (as a percent)			10.00%
Tax Cuts and Jobs Act of 2017, BEAT tax rate, 2026 and thereafter (as a percent)			12.50%
Provision for income taxes related to BEAT			$ 0		$ 0
Provision for income taxes related to GILTI			$ 0		$ 0