Share-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Restricted Stock Units [Member]
Number of Shares [Roll Forward]
Non-vested RSUs, beginning balance (in shares)		3,137,415		3,137,415
Granted (in shares)				109,409
Vested and shares issued (in shares)				(1,491,964)
Forfeited (in shares)				(127,187)
Non-vested RSUs, ending balance (in shares)	1,627,673			1,627,673
Weighted Average Grant Date Fair Value Per Share [Abstract]
Non-vested RSUs, beginning balance (in dollars per share)		$ 13.44		$ 13.44
Granted (in dollars per share)				14.47
Vested and shares issued (in dollars per share)				13.46
Forfeited (in dollars per share)				13.51
Non-vested RSUs, ending balance (in dollars per share)	$ 13.49			$ 13.49
Share-based Payment Arrangement [Abstract]
Share-based compensation expense	$ 1,922		$ 8,971	$ 4,430	$ 28,008
Compensation expense recognized for forfeited RSU awards	488		0	549	0
Unrecognized compensation cost	13,515			$ 13,515
Weighted-average remaining vesting period of non-vested stock (in years)				1 year 9 months
Restricted Stock Units [Member] | Operations and Maintenance [Member]
Share-based Payment Arrangement [Abstract]
Share-based compensation expense	253		260	$ 490	329
Restricted Stock Units [Member] | Selling, General and Administrative Expenses [Member]
Share-based Payment Arrangement [Abstract]
Share-based compensation expense	1,669		$ 8,711	$ 3,940	$ 27,679
Restricted Stock Units [Member] | Minimum [Member]
Share-based Payment Arrangement [Abstract]
Vesting period				10 months
Restricted Stock Units [Member] | Maximum [Member]
Share-based Payment Arrangement [Abstract]
Vesting period				3 years
Performance Share Units [Member]
Share-based Payment Arrangement [Abstract]
Unrecognized compensation cost	$ 32,073			$ 32,073
Weighted-average remaining vesting period of non-vested stock (in years)				1 year 6 months
Performance Share Units [Member] | Employees and Non-employees [Member]
Number of Shares [Roll Forward]
Granted (in shares)	1,109,777
Performance Share Units [Member] | Minimum [Member]
Number of Shares [Roll Forward]
Vested and shares issued (in shares)		0
Performance Share Units [Member] | Maximum [Member]
Number of Shares [Roll Forward]
Vested and shares issued (in shares)		(2,219,554)